Idk, add a new skill to the db or wherever skills are saved and copy/paste the 'Vending' code. Then just change it so that it uses coupons as currency instead of zeny and add it to Merchant cls or make a new item, e.g. the 'Sales' headgear and make it give you the skill (idk about the cart though). Not sure if it's possible that way, though. Almost no idea of C++ or whatever RO is coded in, lol.