the most important part of child support can't be 'bought" by the other party. cash pays for the roof over the head, keeping them warm in the winter, daily food, etc. the other parent should just have that on hand rather than wait for you to get ready to "buy it" like you can do with school supplies or whatever.
so many people think it ends at "ok i took my kid to the mall each month, i did my job" that's not even the majority of it. unfortunately for people's hard feelings, the cost of the kid daily living is commingled with that of whoever they spend the majority of their time with (the kids rent is the bm rent, the kids utilities is the moms utilities). people need to get over that part or get custody