Paperwork is the non-custodian parent's best friend

I strongly beg you to keep all receipts for anything you purchase for your child. If you take the child and buy a coat, keep the receipt until they reach 22. If you take the kid(s) to BurgerKing, keep the receipt. Everything you buy -- prescriptions, school supplies, sporting goods, movies, food, cell phone, cash to kid -- you need to have proof. If you don't have physical proof, it is not work zip. It's also nice to have eye-witnesses to purchases. However, memories can be "fuzzy" over time so I also encourage keeping paperwork. If in doubt, keep it!