If you mean that you want to use the cash credit on your account to buy a membership plan then 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.
You can only do this manually. Membership plan auto-renewal never uses cash credit on your account and always charges the cost of a plan to the registered payment card.
If you mean that you want to use cash credit on your account to buy a plan for your wife via the Gift Shop then, as far as I recall, you can't do that as the only payment option offered is to use a payment card.
If you need to contact 48 support about this then see the links in my signature below.
