I copied the wording from Dave DuGoff's web site and changed it to fit my wash. I think he said it as well as anyone could. Even then, it's still wordy enough that most people won't read it. Here it is.
CREDIT/DEBIT CARD USERS
Our
credit card system allows you to buy as much time as you would like to use. It places a “pre-authorization” or “pending” amount of $20 on your
credit card or debit card account. You may see it on your account. Once the actual amount has cleared, the $20 pre-authorization falls off and you will see the correct charge. This may take more than a day and is a function of the banking system and is not in the control of XYZ Car Wash.