Hello drbboyd@gmail.co 

I need a little more clarification as your statement is a little confusing

When you bill your customers, are you automatically charging them at the same time you send them the bill? Or, are you sending them the bill and relying on them to make the payment?

Reason I ask is because if you are automatically charging them each month and they don't have to do anything (Think a Netflix subscription) then you use sales receipts and there is no need to run a report to see who paid because with a sales receipt, you charge them automatically.

If you send them an invoice and you rely on THEM to pay you every month (think utility bill) then you can simply run a report for any unpaid invoices.

Please let us know what exactly your billing model and we will be happy to help.