I have 10x Cumulus vouchers on which it says EXPLICITLY that you can also redeem them in the online store, but it doesn't work there (voucher is invalid) and the terms and conditions say that if you don't do it directly there it doesn't apply, i.e. no subsequent credits.

The hotline that is officially listed on the website doesn't know anything and wants to send you to a store (I would hardly order online if I wanted to) and I should just take the device there (queuing, tons of people, an hour's loss of time and then a device with a screen diagonal of 120cm plus 13kg by public transport - yay, just what I need on a Saturday).