The hardware store usually has an out-of-stock item that you ask about to see what the associate tells you. Ideally, he will recommend something similar at a similar price. It's very easy to find an empty space on a shelf in some area - I always try the faucets and shower heads. Then you evaluate one department for a particular product using the scenario they give you. Then you check out with a cashier, purchasing whatever you want, but they will reimburse only $1. I'm usually in the store 15 minutes or less.
Initially you will be requesting the shops, and the company will call and assign you. But after you successfully complete some - I think it's 10 but can't remember, it's been so long - you will be able to self-assign. I don't remember there being a limit to the number of locations you can shop on the same day, I don't think there is. You cannot perform more than one shop at the same location on the same day, though.