If you have €10.95 cash credit on your account and want to use it to buy a €12.99 membership plan then you'll need to add €3 more cash credit to your account and then should get the option to pay for your membership using your cash credit at checkout time.
Auto-renewal doesn't use cash credit to pay for membership and you only get the option to pay using cash credit if you have at least enough cash credit on your account to cover the cost of whatever you're buying.
If you need to contact 48 support about this then see the links in my signature below.
