I've shopped the wrong location and not gotten paid, so I know it's painful, and I feel bad for you. It sounds like it was confusing and messy in the request and scheduling, but once the shop was scheduled, you shopped it. You realized when you were entering the shop that the location you shopped is not the one you were assigned.
I would have done what you did, call the scheduler to see if the other location was open. If they could have used the report for the location you did, that would have been a win-win. But, the scheduler said she didn't have an open shop for that location. In that case, yes, I would kick myself and grumble a little, but I would realize, as you did, that I should have confirmed the locations before shopping, and I would eat the $50 I had spent. If it was a restaurant I wanted to shop again, and you clearly have shopped it before and wanted to shop it again, I would get in the car and go do the assigned location, both to live up to my agreements and to keep a good relationship with a company I want to work for again. I hate it when I get careless and it costs me money. It happens.