The way Checkmein does it is via an Custom API (I made one simular Which i can dig out after this post and Contact you . But they way it works is :
Buys Gamepass / Dev Product
Sends a Request to API saying add it to the DB
Sends a Request back to the game.
Then proceeds with the Model.
Then in the actual model (Via Require) They just do it via HTTP checking for the DB.