1. There is no single "the right way", different app have different "right ways"
2. As for the flat fee, it only works initially when things are simple, as time goes on and your business and usecases you support grows, a flat fee won't work anymore.
And for that, with a simplified use case, well, they can scale up to $20k+ a month if they like, that would be ideal, and if they need a more complicated setup or enterpriseyness, migrate off with my blessing and available-not-required hands on support (again for a reasonable fee, maybe $10k if you want the white glove).