Do invoices need to be signed? The rules in the US, the EU, Mexico and Brazil

No. In most of the world invoices don't need to be signed, and a missing signature doesn't make an invoice invalid. US tax law doesn't ask for one. The EU dropped its signature rule for electronic invoices in 2013, and the e-invoicing mandates arriving across Europe between 2025 and 2028 regulate the file format, not a signature.

For the longer answer to "do invoices need to be signed?", look at where the invoice is issued and what you need it to prove.

Exceptions exist, and they're narrower than most guides suggest. All of them are digital. Mexico and Brazil won't accept an e-invoice that isn't sealed with a certificate, because the tax authority's own system checks it before the invoice exists. Spain and Italy require a signature based on a qualified certificate on e-invoices sent to public bodies, and from 2027 Spain adds one to the mandatory B2B e-invoice: if it goes through a private exchange platform, it must carry an advanced electronic signature. And a signature can matter for a reason that has nothing to do with tax: whether your customer agreed to the terms printed on the bill.

Each market this guide covers gets one row in the table. The sections after it explain the rows, then cover what a signed invoice actually proves and how to check one before you pay it.

No federal tax rule asks for a signature on an invoice. The IRS's recordkeeping guidance lists invoices among the supporting documents a business should keep, and wants them to show amounts, sources, the payee and the date. It doesn't mention signing them. Handwritten, typed, PDF or emailed, an unsigned invoice is a valid record.

Where a signature earns its keep in the US is the deal behind the invoice. That's why the stronger move is signing the order or the contract, not the bill.

Usually not. An invoice asks for payment under a deal that already exists, struck when the customer placed the order or accepted the quote. In a dispute it's evidence of that deal rather than the deal itself. Two corners of US sales law make the invoice, and now and then a signature on it, matter more than that.

The $500 rule and the 10-day objection

UCC § 2-201 says a contract for the sale of goods priced at $500 or more can't be enforced unless some writing shows it was made and is signed by the party you're trying to hold to it. A customer's signature on an invoice can be that writing. Between merchants the rule goes further. A written confirmation that the other side receives and has reason to understand counts against them unless they object in writing within 10 days. An invoice naming the parties, the goods, the quantity and the price can work as that confirmation, so silence becomes evidence.

Fine print at the bottom of an invoice

Invoices often carry terms nobody negotiated: a finance charge on late balances, attorney's fees if collection is needed. Under UCC § 2-207 those are additional terms, and between merchants they join the contract unless they materially alter it or the buyer objects.

That's the question the Vermont Supreme Court took up in Bourdeau Bros. v. Boissonneault Family Farm (2020 VT 35). A grain supplier printed "Net 30 days", a finance charge of 1½% a month and an attorney's-fee clause at the bottom of every invoice. The trial court refused the fees, calling the clause a material alteration as a matter of law. The Supreme Court sent it back: whether a term like that materially alters the deal is a question of fact, weighed on things like the parties' course of dealing and whether the buyer was surprised. Years of paid invoices didn't decide it either way.

Here's the practical reading. If a term matters to you, don't count on the invoice to carry it. Put it in the quote, order or contract the customer signs, and check what a late-payment clause can charge before you print one. It's also why it pays to tie the payment to the signed contract rather than chase a signature on the invoice afterwards.

Most people assume EU law requires signed invoices. For electronic invoices it once did, until the rewrite of the invoicing chapter made by Directive 2010/45/EU took effect in 2013.

Article 233 of the VAT Directive sets three duties rather than a method. The authenticity of the origin, the integrity of the content and the legibility of an invoice have to be ensured from the day it's issued until the end of its storage period, on paper or electronically. Authenticity means certainty about who issued it. Integrity means the required content hasn't been changed. Each business decides how to meet those duties, and the article names the ways:

• Business controls that create a reliable audit trail between the invoice and the supply. Matching it to a purchase order and a delivery note is the everyday version.
• An advanced electronic signature based on a qualified certificate and made with a secure signature-creation device, which is today's qualified electronic signature in all but name.
• EDI (electronic data interchange), where the exchange agreement itself guarantees origin and integrity.

The signature is one example among three. Germany copied the structure almost word for word: § 14(3) UStG says a qualified electronic signature or EDI makes an e-invoice's authenticity and integrity count as guaranteed, and any internal control procedure that creates a reliable audit trail does the job too. None of this applies to a quote or a proforma, by the way, because a proforma isn't a tax invoice.

If invoices don't need to be signed, why do signed ones keep arriving?

Public-sector channels, mostly. A supplier that already signs its invoices for a ministry in Rome or Madrid often doesn't bother running a second, unsigned pipeline for everyone else. That's habit, not law. On a private-sector invoice, the signature is still optional.

Between 2025 and 2028 most of Western Europe stops treating a PDF as a valid invoice between businesses. It's tempting to read that as "e-invoices will have to be digitally signed". The official pages say something else. They describe a structured file, XML that software can read without a person retyping it, and a channel it has to travel through. Only Spain adds a signature requirement, and only for B2B e-invoices sent through a private platform.

Germany shows the pattern. Every German business has had to be able to receive an e-invoice since 1 January 2025. Issuers may keep sending paper, or a PDF the customer agrees to, until the end of 2026, and until the end of 2027 if their prior-year turnover was €800,000 or less. The accepted formats are XRechnung and ZUGFeRD from version 2.0.1. The finance ministry's FAQ on the mandate never mentions a signature, so German invoices still don't need to be signed once they turn into XML.

Honestly, the bigger risk for most finance teams isn't a missing signature. It's an inbox that will hold four or five invoice formats at once for the next few years.

Mexico and Brazil are the two markets in this guide where an e-invoice without a digital seal isn't an invoice at all. In Mexico the e-invoice is the CFDI, the Comprobante Fiscal Digital por Internet, and it carries two seals:

• The issuer's seal, made with a Certificado de Sello Digital (CSD) that the tax authority, the SAT, issues to the business. Article 29 of the Código Fiscal de la Federación says this seal "permitirá acreditar la autoría", proves who issued the invoice, and puts it under the rules for the advanced electronic signature.
• The SAT's own seal and folio. Before the CFDI can be issued it goes to the SAT, through the channel the SAT designates, which validates it against article 29-A, assigns the folio and adds its seal.

Article 29-A then lists both seals among the things a CFDI must contain. The printed version a customer may ask for "únicamente presume la existencia" of the CFDI, in the code's own words. It's a picture of the invoice, not the invoice.

So Mexican invoices do need to be signed, just not by a person. Nobody signs one by hand, and a generic signature validator isn't the right tool for checking one. The tax authority checked the CFDI before it existed, so when you buy from Mexico, the useful question is whether the CFDI is genuine and current in the SAT's records. One more distinction worth keeping: the CSD isn't the e.firma, the taxpayer's own advanced electronic signature. You need a valid e.firma to request a CSD, but they're separate certificates with separate jobs.

Brazil's electronic invoice for goods, the NF-e, is an XML file the issuer signs digitally with a certificate from ICP-Brasil, the Brazilian state's public-key infrastructure. The certificate identifies the company by its CNPJ, the federal company tax number. The rule comes from Ajuste SINIEF 07/2005, the agreement between the federal and state tax authorities that created the NF-e.

São Paulo's tax authority describes the effect plainly. The NF-e's legal validity rests on the sender's digital signature, which proves authorship and integrity, and on the tax authority receiving the file before the taxable event. Signing alone isn't enough: the state has to authorise the NF-e before the goods move.

The paper that travels with the goods, the DANFE, is an auxiliary printout of that file. It isn't the invoice.

Fair warning for foreign buyers: you won't learn much by validating an NF-e signature yourself. The state's authorisation is what counts, and an ICP-Brasil root isn't on the EU Trusted List, so a European validator will report the chain as untrusted even when the file is perfectly fine.

Not by default. Under UCP 600, the ICC's rules for documentary credits, Article 18 says a commercial invoice "need not be signed". The catch is Article 1: the rules bind everyone unless the credit itself modifies or excludes them, and Article 2 defines a complying presentation by the credit's own terms. So read the credit. If it calls for a signed commercial invoice, sign it, because the bank checks your documents against the credit, not against what seems reasonable.

A supplier's digital signature on an invoice answers two questions, and only two:

• Who issued it. The certificate names the signer: a person, or a company when the invoice carries an electronic seal instead of a personal signature.
• Whether it changed. Edit one byte of the signed content after signing and validation fails.

Everything else is outside its reach:

1. 1.
   Whether the amount is right. A supplier can sign a wrong number perfectly well.
2. 2.
   Whether the goods arrived or the work was done.
3. 3.
   Whether the bank details are the ones you agreed. They're the ones the signer put there, which is a different thing if the signer isn't who you think.
4. 4.
   Whether you owe the money at all.

A customer's signature is a different animal. It doesn't prove origin. It shows the customer saw the invoice and, depending on the wording and the law, accepted it. That's why some businesses ask for one on delivery: it works as a receipt and an approval on one page, which shortens the argument if the bill is disputed later. It's the UCC territory above, and it's about contract, not tax.

Here's the check almost nobody runs. Open the certificate, read the organisation name and tax number in it, and compare them with the supplier on the invoice and in your vendor master. Payment-diversion fraud works by looking legitimate, and a valid signature from a company that isn't quite your supplier looks very legitimate.

Checking a digitally signed invoice takes a few minutes when the file arrives, and gets harder every month after that, as certificates expire and revocation services go quiet. The order that works:

1. 1.
   Identify the format from the extension. .p7m and .p7s are CAdES, .xml and .xsig are usually XAdES, a signed .pdf is PAdES, and .asice or .asics is an ASiC container. Don't rename anything to find out.
2. 2.
   Upload it to a validator built for that format. Most invoice signatures are enveloped and need only the one file. A detached .p7s also needs the original invoice, byte for byte as the supplier sent it.
3. 3.
   Read the report, not just the verdict. A proper report separates four findings: integrity (the hash still matches), the certificate chain (does it reach a trusted root, such as one on the EU Trusted List), revocation (was the certificate revoked when it signed) and the timestamp.
4. 4.
   Compare the certificate's subject with your vendor master, as above.
5. 5.
   Archive the original file and the report together, and only then extract the data for your ERP.

When a signed invoice fails validation

A failure isn't automatically fraud. The commonest cause is handling: a mail gateway or an archive re-saved the XML, and a reformatting pass can break the digest without changing a single visible character. Ask the supplier to resend the file as a direct attachment and run it again. If the second copy fails the same way, you've got a conversation worth having before the payment run.

Chaindoc's signature verification takes XAdES, CAdES, PAdES and ASiC files through one form, validates them with the EU's DSS library against the EU Trusted List and gives you that four-part report. You confirm your email address with a one-time code first. The file you upload isn't stored, so save the report when it opens and file it with the invoice.

Chaindoc isn't an e-invoicing provider. Its payments feature can generate an invoice from a signed contract, but it won't produce an XRechnung, a CFDI or an NF-e, it isn't a French plateforme agréée, and it doesn't report anything to a tax authority. If your obligation is a structured e-invoice, that's a job for invoicing software connected to the right channel.

It helps on either side of the invoice:

• Before it, with the quote, order or contract your customer signs, which is where payment terms belong. The amount can ride on that signed contract and be collected through Stripe when the client signs.
• After it, by checking a signed invoice you received with the verification tools above and keeping the report.

So, do invoices need to be signed? In the US and across most of the EU, no. In Mexico and Brazil, yes, by the tax system's own certificates. For public bodies in Spain and Italy, yes, on a qualified certificate, and in Spain from 2027 also on B2B e-invoices sent through a private platform. And when the real question is whether your customer agreed to your terms, get the contract signed, not the invoice.