Not trying to be sassy but… probably for the better – Payback, like Deutschlandcard, are classical carrot-on-a-stick schemes to milk you like a cow. They collect and cross-reference purchase data and sell them to all kinds of actors, while the collective savings you get by accumulating their points is a very tiny fraction of what they "earn" by doing so. It's not only nonsensical to use a VPN with them since they already know who you are (and now probably know you like VPNs), it's even hazardous from a data economy's point of view. You should steer clear of such schemes.
In any case, good to know they do so. I'd extend this warning to deutschlandcard.de, too.