Flexible shifts hiring in San Diego - any advice?

A grocery chain and a hotel near San Diego are hiring part-time staff. There are 10 people counting on me, so I am trying to be careful. Shifts are four to six hours and the pay is weekly. Is the schedule really fixed?

Keep everything in writing and save the confirmation numbers. It saved me later.

Thanks for writing this out. I went through something close to it last year, and it got easier once I kept a simple log of every call and form.