When I made the change from post pay to pre-pay, I gave the families 30 days to get ahead and be on a pre-paid schedule with all payments due on Fridays prior to the next week of care. They can pay weekly or bi-weekly. I used the excuse that I am using a payment program for accounting purposes much like Quick Books. I told all my parents that my late fee is $5 per day for any payments not paid by Friday at 5:30 P.M. (weekends included) and that my computer program automatically adds the fee if payment from the parent is not entered on time. I told the parents I am sorry for any inconvenience it causes but it makes accounting so much easier for me. This also makes it impossible for parents to try and ask for extensions on paying or the old "i forgot my check book..." excuse. I just smile and say, "No problem, but a late fee is automatically entered." I also let them know that I cannot alter the program. So pay up on time or make sure you include the late fee when you do pay. I can honestly say I have not had one single problem with late payers.
Just blame the program. Parents can't argue and take advantage of your human feelings.