I was thinking, and this is what I came up with. How about for the Post Points, have one of the shop items be x amount of reputation points that non-donors can use. Just a thought.
Implementing that would require a lot of recoding of MYPS and the reputation system, if you want to code a plugin that can do these things by all means do.
Im currently working on a shop that will use myps points, however it wont be implimented as a plugin. If that what you meant because i couldn't understand OP