It's all about record keeping and it's why you have overhead when you think you don't. You otta keep records no matter how the customer pays. You should get paid for record keeping just like working in the field. The customer should have a canceled check, cash receipt, money order receipt or credit card slip for each bill you give them. If you say they didn't pay for a certain invoice, and they should be dated and serial numbered, and they can't provide proof of payment, tough. Burden of proof is on them. Didn't anyone ever ask you guys for proof of payment?