What do you think of my plan to ask my girlfriend to buy me dinner in exchange for going with her to buy a car?
Do you think that's fair?
All I can tell you is that I'm very sick of buying her dinner all the time.
I can see why you would be annoyed at having to buy dinner if she never offers to. That's not a reason not to go with her to go buy a car though. She is your girlfriend and she wants your support and help in this important decision so you should try to be there for her.
What do you know? In the meantime I am just going to refuse to help her if there's a problem with her car.
If your girlfriend's new car develops a problem, you should absolutely help her. Refusing to help her out of spite for her not paying for dinner will likely lead to issues in the relationship. It's important to help your girlfriend!