As a Roblox user when I buy builders club I pay both on card and in credit, but as you can see they is always some credit left over. 
And the cheapest builders club is £5 (British pounds)
My idea is to have the option to pay with the credit and a debit/bank card as this could save users money and wont have the credit left over in the account so the users doesn’t have credit which they cant really spend as they isn’t no item for this price.
The option would look this this and have the option for bank cards as well
Hope this gets taken into consideration!