A billing invoice is a document that small companies, independent contractors, and freelancers use to charge for their services or products. Billing typically occurs monthly, although a company can use this invoice for one-time charges as well. Although the customer is responsible for paying the bill by the due date, the company needs to notify them with an invoice on time. This step prevents the customer from having their water or utilities shut off. Both parties need to mark the invoice as paid after the transaction occurs. Each person should retain a copy of the form as proof of payment.