Good idea Virus. I agree an "accepted" proposal or other form of written agreement is a must. There are times when the profit on a job is small and one has to ponder how much to spend collecting an account. You could (and should) place the following clause in your proposal/agreement. "Terms are: net upon receipt of invoice. Invoices past due are subject to 2% monthly interest and service charge. Invoices past due 30 days are subject to collection and customer agrees to pay costs of collection including reasonable attorneys fees."