You cannot pay child support in California just by giving your ex cash and telling him or her that it's for a support payment. The law says that everyone in the state who is paying support actually needs to send it to the State Disbursement Unit first, and then it is sent out to those who are to receive it. This makes payments easier to track and verify.
There are three main options that you can use if you're getting payments, with the first being a bank check that is simply mailed to you. This is easy, it doesn't rely on technology, and it's perhaps the simplest way for many people to get their money. You do have to cash the check before it can be spent, though, and some banks having waiting periods before a check will clear.
The second option is to use an EPC -- an Electronic Payment Card. This functions similarly to a debit card. The money is sent to the SDU and then they put it on the card. You can access is quickly, you don't have to worry about losing cash, and you can spend it as you would money from a standard debit card.
Finally, you can use Direct Deposit. The SDU will just send your money to your bank account with an electronic payment. After it clears, you can spend it electronically or go to the bank and take money out in cash. Direct deposit saves you a trip to cash a check. If you just buy everything on a credit card and then pay it off online anyway, this is an easy way to go.
As you can see, each child support payment option has its own advantages, so it's important to consider which one would be best for you.
Source: California Department of Child Support Services, "Custodial Party FAQs," accessed Aug. 23, 2016