Do you have enough cash credit to cover the cost of your membership plan?
If not then you need to add more until you have.
If you already have enough then you should get the option to pay for your membership plan using your cash credit rather than payment card at checkout time.
The real sweetener is that you can purchase your plan from your available credit balance, so long as the balance covers your full plan cost.
Do you not get the option to pay for your plan using your cash credit balance here?
If not then you'll probably need to contact 48 support for assistance.
See the links in my signature below.
