Heyo,
There's a site out there by the name of "Midasbuy" that does legit top-ups for many (most) mobile games. They are literal affiliates for many games and can read into their servers as the items you purchase will be delivered right away into your game account by the time a purchase is confirmed.
This being said, they offer a 1+1 offer upon the purchase of coupons, for example on the Age of Empires Mobile game, but that applies ONLY for the first buy. For example: you buy a €99,99 coupon, you will get another one for free, only for the first time.
The way it works is, through a GET command which is the "getDrmInfo" they fetch the user data, in-game, and subsequentially after a server check, the website automatically gives whether you already bought the offer or not, therefore enabling or disabling the bonus coupon.
If the fetch command is blocked (along with the tampering of few other functions in order to unlock the page after an error message) the coupon stays, and you can almost proceed to a purchase before getting an "unable to proceed with the payment" message. Still not sure if it's anything server-sided and if the blockage of said functions really prevents 100% the purchase to go off.
Would like to know if there's actually anyone, way more experienced than me (I'm somewhat a beginner), with HTML tampering that manages to get it working (i.e getting unlimited +1 coupons while still paying).
I can provide several usernames+server IDs (needed for testing and fetching of infos before a purchase is made) if anyone want to give it a shot.
Obviously willing to pay well for this.