Nicole, 
I haven't thought about pricing , but the idea of it being a charity event sponsored by the pet store does not mean you have to give your time away.  You were hired to do the shoot and provide a service.  If you want to donate your time for the rescue group that's between you and the rescue group.
The other thing.  Your table should be your table. I would have asked store management to provide another place for the tip jar. Not cool!
Doug