Is it a must when selling devproducts or gamepasses? Would you provide a small example?
http://wiki.roblox.com/index.php?title=Developer_product
As listed in the wiki, all dev product purchases need to have the receipt processed. In the code where you process the receipt, you add the code that grants the user the item(s) that they purchased.
Then, your function MUST return Enum.ProductPurchaseDecision.PurchaseGranted in order to be credited for the dev product.