FAQs: Online Shop
Vouchers which are not credited to the shop account cannot be redeemed:
Vouchers which have already been credited to your customer account cannot be redeemed in the shop, or cannot be taken into account during invoicing.
On payment via bank debit entry, the credit amount is automatically taken into account on the debit entry sum, as can be checked on your account statement.
On payment via transfer, the credit amount can be deducted from the transfer sum.
Here you can find the "Customer account"