I don't issue invoices either. I expect payments to be made without any reminders. However, I purchased a receipt book from the dollar store and whenever a payment is made I write out a receipt in duplicate indicating the date, who paid, with 'childcare for -----' and the dates that they were in care eg. Aug. 20-24 and my signature. At the end of the year I tear out all the duplicates and give them to the parents all at once with a typed summary and total for their records to match all the receipts.