Open MyPostcard once via your browser and log in with your normal MyPostcard account. Now you can click on the desired product under "My Orders" and click on "Download Invoice". Then you only have to enter the billing address and you're done.
The whole thing also works with credit. If you click on "Recharge Credit" in the list on the left, you will see an "Account overview" under the credits. Next to each amount is a small note. There you can click on and you will be asked again for your billing address.
For better visualization there is also a PDF added .