I had a developer create 2 modules for me- 1) license code "bucket" distribution system and 2) Amazon fulfillment. I am extremely pleased with the results and have been using both modules on a production site selling security solutions to large enterprise customers. She collected rather modest fees for the module (under $500) which I think was well worth it.
You can contact her via the Contact Me box at
http://modules.terrahost.org/x-cart/x-cart-module-fulfillment-by-amazon/ or directly at
sbankova@gmail.com
Good luck!