I've played an other game something like a year ago which had a guarenteed "chest" aswell. If it is similar to that one it works like this:

Buy 40 chests > Reward tier 1 (something like 20 Fusion Boosts)
Buy 40 chests again > Reward tier 2 (something like Guarenteed older epic)
Buy 40 chests 3rd time > Reward tier 3 (something like guarenteed Epic+)