@btemps
This shop is not just for food. Yes, you get to keep what you don't spend. Some of us have several hundred $ in gc now. I used some to host a superbowl party and had it catered by BJ's. That is real money that I did not have to spend buying inferior pizzas at Pizza Hut or Papa Johns.
Yes, it's not a shop payment like doing a cell phone shop or a gas station shop, but i love the more money I can accumulate with the GC. Looks like in your example, you netted $30.
There are all kinds of things you can do with the GC to "monetize" it or you can just keep it and feed yourself without the shops.