You mean that you added €20 cash credit to your account?
Why don't you use it to buy your next €12.99 plan and you'll still have the other €7.01 left for other purposes?
You could then add another €5 to buy your following €12.99 plan from the resulting €13.01 cash credit balance.
If you really need a refund then you'll have to contact 48 support to see if they can assist.
See the links in my signature below.
