How do I send an invoice to a large company's accounts payable department?
How to send an invoice to a large company's AP department correctly
Short answer
Large company AP departments are workflow-driven: your invoice must include a purchase order number, be sent to the correct email address or portal, and match the PO exactly in amount and line items. Without a PO number, most large-company AP systems automatically reject or hold the invoice — before any human sees it. Get the PO before starting work.
Service businesses usually don't get paid by large companies because the company refuses. They don't get paid because the invoice never made it through AP intake. Big-company AP runs through automated systems that check for a purchase order number, confirm the vendor is approved, match the invoice amount to the PO, and route to the correct cost center for approval. An invoice missing any of those elements may never reach a human.
Before you start work for a large company, get the purchase order number in writing. A PO is your proof that procurement or finance has approved and budgeted the expense. Your invoice has to reference the PO number explicitly, usually in a field labeled "PO Number" or "Reference Number." The AP system matches that number to the budget authority and the amount to the approved spend. A mismatch by a few dollars (rounding, shipping, anything) can put the invoice on hold.
Find out how the company actually receives invoices. Many large companies require submission through a vendor portal (Ariba, Coupa, SAP) instead of email. Others accept email but only at a specific AP address that is not your contact's address. Sending the invoice to your project contact rather than AP is one of the most common sources of payment delay. They may forward it eventually, but it can sit for weeks. Ask directly: "Where should I send my invoice and what information do you need on it?"
Format matters too. Large AP departments often can't process PDFs embedded in emails. They need the invoice as an attachment, in a specific format, with specific fields filled. Some require an invoice number matching a specific pattern. Getting this information upfront, ideally at contract signing, saves weeks of follow-up later. And always confirm receipt. A short email to your contact ("I submitted invoice #1234 to AP on May 1 — please let me know if anything is needed") keeps the payment on someone's radar.