Mohamed,
By charging interest, you are required to follow certain specific rules and laws. It varies by state. It involves the extension of credit.
What may be a legally safer option is to charge a flat "Repeat Billing Fee." The fee would be the same regardless of the amount of the bill. At least in primary care, most bills are not really high, so if you have a flat re-billing fee of, say, $5, it will be higher than any interest rate anyone would likely come in contact with (unless they borrow from an investment bank or loan shark.) Since you are not charging interest, this would avoid laws concerning the charging of interest and extension of credit.
