I can now explain the confusion:
During a promotion period in Germany, all standard delivery fees have been waived (for a couple of months). That promotion period is over now.
So, the charges are in fact:
First card = 0 € plus 6 € delivery
Second card = 6 € plus 6 € delivery
Since the delivery fee has been waived during the promotion, people paid:
First card = 0 € plus 0 € delivery (promo)
Second card = 6 € plus 0 € delivery (promo)
That is the source of the confusion for people in Germany. 