Stocard are no longer offering payment functionality via a dedicated card of their own, according to the change log of their latest update to the app:
We’re saying goodbye to payment. Stocard is still here. We’re just focusing on helping you to collect your loyalty cards in one app while working with Klarna to bring you more smoooth ways to pay.
What a shame. This probably explains why I wasn't able to find any reference to payment a month ago.