Review Their Invoices
When you receive an invoice from a vendor, verify it immediately. It's not uncommon to find mistakes on invoices. Typically, you'll have at least two other documents to compare with your invoice: your original purchase order and the product packing slip. Make sure every number on the source documents match the invoice, including quantity, unit price, extended price, sales tax (if applicable), and freight or delivery charges.
Once you're satisfied, you can file the invoice for payment. If the vendor offers early payment discounts, flag the invoices to make sure you remember to pay within the discount period. Lacking that price benefit, pay on time but don't pay early; if the terms ask for payment in thirty days, take thirty days to pay, as that maximizes your cash-flow capabilities.
Should there be any question about the accuracy of any invoice, contact the vendor immediately. Have any applicable purchase orders or packing slips readily available, and send over copies if needed. To keep things friendly, it's a good idea to pay any other open invoices while this one is being resolved. Also, if there are other items on the invoice that you don't dispute, you can offer the vendor a partial payment on the invoice.