What happens when you try?
Any error messages?
Do you have at least as much cash credit as the plan that you're trying to buy costs?
If not then you won't get the option to pay for stuff using your cash credit balance.
For example, if you had €10 cash credit on your account and wanted to buy a €12.99 plan then you'd need to first add another €3 cash credit to your account (giving you a €13 cash credit balance) before you'll get the option to pay for the €12.99 plan using your cash credit balance.
If you need to contact 48 support about this then see the links in my signature below.
