You are right to be careful with your credentials. Our model needs access to the game account so we can make the purchase for you, so here is exactly what that means.
We use the login you added in the app to sign in to the game, buy the packs you ordered, then sign out and remove the account from the device. We access nothing else. Please check the full list in What Packsify never does inside your account.
If you would rather not use your main account at all, you do not have to. See Can I use Packsify without sharing my main Gmail or Facebook password.