If the amount of cash credit on your account is at least equal to the cost of your first membership plan (and one time activation fee?) then you should get the option to pay using your cash credit instead of using your payment card at checkout time.
See here:
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.
If you need to contact 48 support about this then see the links in my signature below.
