How do I acknowledge and document her payment of 50% of the unreimbursed medical bills the proper way?

I provide the health insurance coverage for my daughter and our orders state that each parent is responsible to pay 50% of all unreimbursed medical bills. I pay the copays since I hold the policy. I get the bills and pay them. When I asked her for her half, she pays it. Every form for the affidavit for direct payment states that the custodial parent has been paid by the non custodial parent for such bills. Do I just give her a hand written and signed receipt when she pays me? Is there a specific format or an attorney general’s office form that I need to use so that she has a receipt and knows I am not going to go back and screw her over say she never paid me?