Check your state laws to see how much you can legally charge.
From my contract:
PAYMENTS & DISPURSEMENTS: Unless stated otherwise, Client will be billed on the first day of each month for the prevous months service. Payment is due upon receipt. Payments received after the 15th of the month will be charged a $5 late fee. If payment is not received by the last day of the month, service will be suspended until payment is received. Once the amount is paid in full, work will then continue. Any extra cost required to bring the property back to workable condition shall be billed to the customer at $40 per man-hour. A service charge of $25 will be charged for any returned check. Should it become necessary to bring legal action to collection amounts due under this agreement, Client agrees to pay all costs of such collection including reasonable attorney’s fees and court costs.
You could start charging a finance charge for customers who's payment is recieved after the due date. You should give notice to your customers first. We allow our customers 21 days to pay their bill..with a 3 day lean for those late ones coming in. If you are going to charge an interest charge make sure to check with your state for the right amount that you can charge. In my state if you are INC you can charge 17% annual interest, but if not only 7% annual.