It's actually even simpler than that. If we make the in-game currency some arbitrary thing (like gold in other MMOs, we went with Credits for the sci-fi theme), then we can easily have you buy a pack on Steam. We don't ever even see your credit card. No need to ever charge our customers directly.
Ah, i see, because i was worried you'd be taking CC details form customers and thinking securing them wouldn't be important.
I reckon you should still do an in-game crypto coin, just pick one of the others. Risk of attack can be mitigated by accessing the wallets only in game, shirley?