Manufacturers selling to larger companies live inside their customers' purchasing systems. The customer's PO number, line numbers, item codes and units have to appear on the invoice exactly as the customer issued them, or the invoice fails their three-way match and waits.
Their item codes, your products
The customer calls your part “BRKT-22-ZN”. You call it “Zinc bracket, 22mm”. Both names need to live side by side: yours for your catalog and your ledger, theirs for their invoice. Learning that mapping once, per customer, saves retyping it on every invoice.
Track what each PO line has left
A blanket PO is a budget. Every shipment draws it down. Before an invoice goes out, you should know how much of each PO line has been billed and whether this invoice would go over.
Quote, PO and invoice should agree
When the customer's PO cites your quote, the prices on the PO should match the quote. When they don't, that is worth catching before you ship, not after you bill.
Required fields are per customer
One customer wants a buyer name on every invoice. Another wants a cost center. Keeping those rules per customer, and checking them before an invoice is approved, avoids the rejection email.