Commission App With Variant Level Capability

[LadyKM]

Hi - does anybody have suggestions for a commission/royalty app that works at variant level. E.g If we sell a small, medium or large print of the same design, I can allocate a flat rate commission (not %) to the artist. I've found 'Goaffpro' does variant level, but feel it is expensive, as I only need this small feature of this programme and not all the affiliate parts!