A collections person may find that a customer is unwilling to pay an invoice until a proof of receipt can be supplied, showing evidence that the customer received the goods. The customer should be able to obtain this information itself, but its processes may be so poor that the accounts payable staff cannot obtain this information from its own receiving department.
If this scenario is causing payment on some invoices to be delayed to an inordinate extent, consider using one of the package delivery services to obtain the information. For example, shipments sent by UPS or FedEx can be set up to require a receipt signature by the customer. These shipping companies post the signature and related receipt information on their web sites, which the seller can then forward to the customer's accounts payable department. Alternatively, you can include the receipt information and signature image directly into an invoice, though doing so means that invoices are only issued as of the receipt date, rather than the more common delivery date (which can also delay the recognition of revenue).
Using this approach to obtaining proof of receipt means that a seller is restricted to using only those shipping companies that provide this type of receipt evidence, which may be among the more expensive shipping alternatives.