Be sure to follow invoicing to the letter of the customer’s instructions, even if they are cumbersome or payment might be significantly delayed.
Typically, you will need to cite their purchase order number and mail the invoice separately from the shipment. Often the purchase order number will need to be included on the shipping label or in documentation accompanying the order. If the purchase order is missing, some firms may refuse to accept delivery of the order. Many large firms have instituted draconian penalties for not following shipping instructions precisely.
In addition to sending an invoice at the time of the order, you should send each customer a statement each month that summarizes account activity. Statements should include outstanding invoices, payments received during the month, and any other credits or charges. If you are a very small firm dealing with other relatively small businesses, an alternative to mailing statements may be to send copies of any outstanding invoices marked “reminder” or “second notice.”