Hard Copy Invoice Requirements: When a Customer's Accounts Payable Office Wants It Mailed

Accounts payable offices that want a hard copy invoice ask for much the same things: their PO number on your invoice, your invoice number and date, a remit-to address, itemized lines, and one copy mailed to the address their vendor page or PO names. That's usually Accounts Payable at their bill-to address, not the person who placed the order. Many also take a PDF by email; where their page says to use one route, pick one, not both.

The list below comes from 15 AP pages. Your customer's own page or PO is the one your invoice is checked against.

What accounts payable offices require on a hard copy invoice#

Seven items come up on page after page:

Where the official guidance lives: your customer's own vendor or supplier page, or the billing instructions on their PO; if you can't find either, ask whoever placed the order for their AP contact. The table draws on pages like these, each checked October 14, 2026:

• California Department of General Services, Invoicing Guide for Suppliers (opens in new tab) — a state agency's full list, with paper rules: clean black-and-white print, no staples, one copy.
• Children's Hospital of Philadelphia, Invoice Submission Requirements (opens in new tab) — a PO number on every invoice, and email or mail, not both.
• Wayzata Public Schools, Accounts Payable (opens in new tab) — a school district's PO rule, "Attn: Accounts Payable" address and payment terms.
• Harris County Auditor's Office, Invoice Requirements (opens in new tab) — a county's field-by-field list, from the remit-to address to the payment due date.
• City of Columbia, Missouri, Accounts Payable (opens in new tab) — a city's mailing address, PO rule and statement policy.
• Washington University in St. Louis, Vendor Invoice Requirements (opens in new tab) — mail to Accounts Payable, not the sold-to or ship-to address, with one PO per invoice.
• Stanford University, Submit an Invoice to Accounts Payable (opens in new tab) — invoices without a valid PO number are discarded.
• UCLA Health, Payment and Invoice Information (opens in new tab) — unique invoice numbers, your legal name, a remittance address and your tax ID.

How to mail a hard copy invoice to accounts payable#

However you send it, the job is the same: the right fields on the page, one invoice per envelope, addressed to Accounts Payable. With Postmarkr, which prints and mails USPS First-Class letters to US addresses, it's five steps.

1. Put the PO number, invoice number, date and remit-to on the invoice. Add them in your accounting software and export one PDF per invoice; how to mail invoices from QuickBooks Online, Xero, FreshBooks or Wave shows how in each. Supporting pages the AP office asks for, such as a timesheet or packing slip, go behind the invoice in the same PDF.
2. Upload it and address it to Accounts Payable. A typed address doesn't have a separate company line yet, so type "Accounts Payable" and the company name in the Name field, up to 40 characters. Use the address on their AP page or the PO's bill-to block, not the buyer's office or the ship-to address. It's checked against USPS address data before you pay.
3. Check the preview. The address page comes first, placed so their address and yours show through a #10 double-window envelope; your invoice never has to line up with a window. Behind it, the invoice prints as you made it. Our print partner adds a small code strip in the bottom-left margin of every page, and black-and-white letters print in grayscale. Check the address page in your preview before you pay, and if their AP address has extra lines, such as a mail code, make sure each one fits.
4. Send one invoice per letter. A PDF goes to one address and isn't split, so a file holding several invoices puts them all in one envelope. For several invoices or AP offices, add each PDF to your Cart with its own address and pay for up to 10 together. Bulk Send is for one document to many people, not different invoices.
5. Pay by card. A one-page black-and-white invoice is $2.05, all in: printing, envelope, address page, First-Class postage and fee. See totals for longer invoices. Printed the next business day.

The address check matters. Mail returned as undeliverable because of an incorrect or incomplete address is not eligible for a refund or free resend. If USPS confirms a letter was lost, we'll resend it for free. There's no return envelope yet, so the remit-to on your invoice, plus a pay-online link if you have one, is how payment finds you.

Printing it yourself? How to mail a paper invoice covers window placement, folding and postage; one stamp covers about four sheets of printer paper with the envelope.

Send it once: mail or email, not both#

Most AP offices that take paper also take email, and those that mention duplicates say to choose one route per invoice. Children's Hospital of Philadelphia's page puts it plainly: "Please use ONE of the methods below — if you use both, it will delay payment." California's DGS asks suppliers to avoid sending the same invoice through more than one channel, and Bentley University (opens in new tab) and Huntsman (opens in new tab) ask vendors not to send copies unless they request one, to prevent duplicates.

• Emailed it to an AP office whose page says one route? Don't also mail it unless they ask. Sending a paper backup where no such rule is published? Label it as a copy of the same invoice so it isn't entered twice.
• Asked for paper after an email? Mail it with the same invoice number and a line saying it's a copy of the one you emailed on that date.
• Never give a resent invoice a new number; it can be entered and paid as a second bill.
• If the page asks for a copy somewhere else, send it and label it. The City of Columbia, Missouri, wants the invoice at Accounts Payable and a copy with the department that ordered.

When a mailed invoice counts as received#

Where an AP page says when terms start, it's usually the day they receive a correct invoice, not the day you date or mail it. Wayzata Public Schools' page says: "We pay net 30 days from the date we receive a correct invoice, unless your contract states otherwise." California's DGS calls that date Day Zero, and Old Dominion University (opens in new tab) counts 30 days from a proper invoice or from the goods or services, whichever it receives later. Stanford's example counts from the invoice date instead. Your PO or contract says which applies.

On the pages that say so, received means received by Accounts Payable. DGS warns that invoices sent to buyers or individual staff can delay the official receipt date, and Washington University says sending to the sold-to or ship-to address may delay payment.

For a mailed invoice, add it up:

1. You mail it: at a Post Office, before that day's last pickup; with Postmarkr, it's printed the next business day.
2. USPS estimates 1–5 days (opens in new tab) for First-Class delivery.
3. AP logs and enters it; Michigan State University (opens in new tab) says entry takes about a week after receipt.
4. They pay on their terms, and a mailed check makes the trip back.

That's arithmetic, not a promise. For a worked count back from a due date, see how to mail a paper invoice.

What to keep, and what your record can't show#

Keep these together for each mailed invoice:

• The AP page or PO instructions you followed, with the date you read them.
• The invoice PDF exactly as you sent it.
• The date you mailed it and the address you used.
• Any reply from Accounts Payable, including a note that it's entered.

First-Class letters aren't eligible for USPS Tracking. If a contract or your customer asks for a USPS record that the invoice was mailed, buy a Certificate of Mailing at a Post Office counter when you mail it: $2.45 for one piece, issued only at the time of mailing. For a delivery record, Certified Mail is the USPS service; Certified Mail alternatives compares what each option records.

The best receipt for an AP invoice is AP's own: ask them to confirm it's entered, and note the date and who told you.

A cover note for the envelope#

A short note in front of the invoice tells AP what's in the envelope and whether it's the only copy. Make it the first page of the same PDF; with Postmarkr, a one-page invoice and a one-page note in black and white come to $2.26. Or type the same lines on the invoice, above the total. {{double_braces}} change with each invoice; [square brackets] are wording you write once.

To: Accounts Payable, {{customer_organization}}
Re: Invoice {{invoice_number}}, PO {{po_number}}

Enclosed: invoice {{invoice_number}}, dated
{{invoice_date}}, for {{amount_due}}.

[Keep one option and delete the other.]
This is the only copy we are sending.
[or]
This is a copy of the invoice we emailed on
{{email_date}}. Please enter it once.

Remit to: [your remit-to address]
Questions: [name, phone, email]

Questions vendors ask#

Do they need the original?#

Three of the AP pages we read ask for one (Broward County (opens in new tab), the City of Columbia and Bentley University). None says what that means. If it matters, ask their AP contact what they accept, then send one copy, once, matching the PDF you keep.

Can I email and mail the same invoice?#

Where the AP page says one route, only if they ask; otherwise two copies can be entered and paid twice. Where it doesn't, a paper copy is fine if it carries the same invoice number and a line saying it's a copy of the one you emailed, with the date; that's the copy line in the cover note above.

What if they say they never got it?#

Ask who enters invoices and at what address, and check the PO's billing instructions. Some, such as Huntsman, want a help-desk query rather than a second copy. If the office asks you to resend, send it once, with the same number, labeled as a copy. If email keeps failing, see why QuickBooks invoices land in spam. A mailing record shows what went out and when, not that AP entered it.

Can I put several invoices in one envelope?#

Mail each invoice as its own letter. The AP pages that cover this want each invoice in its own file. An invoice's supporting pages, such as a packing slip, can go behind it.

Can I send a statement instead of the invoice?#

Usually not, for payment. California's DGS doesn't accept a statement of account as an invoice (utilities and financial institutions aside), the City of Columbia pays from one only when a vendor doesn't issue invoices, and Huntsman wants statements sent to its help desk, not where invoices go. Send the invoice, and send statements the way they ask.