My idea as of right now is to have a website for your game or an official paypal link, etc. When the game recommends to a player they needs to buy “gems” or whatever a user can press a UMG button to open a webpage. There are nodes for this in blueprints, one is called “launch url” so you suggesting that unreal is “behind the times” is not really correct. You just need to critically think about a solution.
So the user follows the link to buy gems… Upon a successful purchase, your webpage will give the user a key to enter into the game. So back into the UMG in game, the user enters the key, your blueprints already have valid keys and a system to check to see if the entered key is a valid one, if it is add the gems and make sure that code is no longer able to be used again. You feel me? Might take some work, sure but it is possible to make a system in blueprints.