@arise77I've also had an issue with a card being charged despite being shown as reimbursed in my account history.
That is how it worked last time - as the name implies it was, and is, a "cashback", not a "buy three get one free" offer. Last time you had to pay for the fourth card and then get the credits refunded. This meant that the account history, quite correctly, showed the credits being taken. It also meant that if you did not have enough credits to pay for the fourth card you could not buy it until you first topped up your credit balance. I assume it works the same way this time.