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.