Nathaniel,
You need to be VERY careful about the way you handle this. This cannot be an endorsement of any kind. When you say advertising, it sounds as though your PTA would be promoting these businesses and that is a very big NO NO in PTA. You can thank them for their contributions and acknowledge them but it can't appear to be an endorsement.
However, if you just mention that if you make a purchase at one of these establishments and a portion of it goes to your PTA, that would be fine.
You also need to make certain that you make this opportunity available to many in the field.
For example, if you use a car lot, you need to put this offer out to many car lots. It can't be exclusively offered unless the car lot itself offers this opportunity to all schools.
I would strongly suggest you contact your Council or State office on this one. (I'm a NV Council but the advice you get from CA may be a little different)
If you need help locating who to talk to go to
www.pta.org/ptacommunity/localptas/ptalinks.asp?div=California
Critter hits it right on the head for other reasons. Unless it's a proven fundraiser for your group, I wouldn't put all your effort into it but rather try it as a sideline. If you need an example of how this can fluctuate, take a look at your Target and Scolari's checks for the last couple years. (And they both do the work for your group, receipts etc. They just cut you a check)
You are right to question this, Good luck.