One way to handle it is when making the appointment, say you’re only coming if you can pay, at least this one time.
I agree with the consensus here that you should continue to let her pick up the tab for lunch because that’s what makes her happy and in turn, you should come up with something homemade to make you happy. At least, that’s how we do it in my family with one of our friends who is the exact same way.
Good luck!
If it aint broke don’t fix it.