Does anyone know of tutorial for writing a storekit plugin?

I do not want to use prime31’s plugin.

I doubt that many people have written a tutorial - it’s a relatively complex subject, and I can’t imagine that there would be that much of an audience for it seeing as most folk wouldn’t want to recreate something from scratch when there are a lot of good alternatives already out there.

If you really want to recreate the code yourself, why not browse the repository? (although, I would strongly recommend you simply implement their client as is)