If my payment logic is getting too coupled, I can easily extract it because I use one migration file per model and separate code using regular Python packages.

If my payment logic is getting too coupled, I can easily extract it because I use one migration file per model and separate code using regular Python packages. I can easily extract the logic into it's own app if I need to publish it to PyPi for others to enjoy as well. Of course, if you code is getting coupled, then you need to revise your design process.