Distributors bill from three documents that rarely agree on their own: the customer's purchase order, the warehouse's packing slip or delivery note, and the price list or contract that says what each item costs. The invoice is where those three get reconciled, usually by one person with all three open in different windows.
Units are where most errors start
A customer orders by the case. The warehouse ships some cases and breaks one open to ship loose packs. The price list quotes per pack. The customer's own system calls a case a “CTN”. Each of those is reasonable on its own. Put together, they turn a simple line into arithmetic, and arithmetic done by hand at month end is where a 12 becomes a 10.
A careful process keeps each unit as written on the document it came from, and converts only through a factor someone has confirmed for that product. “1 case = 12 packs” for one item says nothing about another item that ships 24 to a case. Guessing a conversion is how an invoice ends up off by a multiple.
Partial shipments should bill when they ship
When 120 of 200 units ship, most customers expect to be billed for 120 now and 80 later. Waiting for the order to complete delays cash. Billing the full 200 creates a credit note later. Each delivery should become something you can invoice on its own, tied back to the PO line it fulfils, so the second invoice knows exactly what the first one already covered.
The PO is the customer's ceiling
Larger customers match every invoice line to a PO line before they pay. If the invoice bills more than the PO authorized, or names an item the PO doesn't have, it goes back to you. Checking against the PO before the invoice leaves is cheaper than finding out from their AP team 30 days later.
Price disagreements are questions, not edits
When a PO shows a price that differs from your price list, someone has to decide which one is right. Quietly overwriting either one loses information. The useful answer records both prices and who chose which, so the next invoice for that customer doesn't repeat the conversation.