I still don't get it.
When you topup your cash credit balance with a topup voucher you do so with the voucher code.
Once that's done the cash credit should appear on your 48 account's cash credit balance and the voucher code is no longer relevant (because it has been used).
Thereafter you can buy membership, add-ons or paid services with the cash on your account's cash credit balance.
I still don't understand why you need the voucher code again.
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Hope this helps.