Amazon App 15 euro credit does not work?

There's this action again that you can get 15 euro credit in the app when logging in and then use it from 30 euro order value. I wanted to get a PS4 controller cheaper with this 15 euro credit because I still have 60 euro credit balance plus theoretically this 15 euro. With the payment options, the 15 euro voucher is not shown to me, until I have something much more expensive in the shopping cart. Why is that so? The controller costs well over 30 euro! What can I do to make it work with the controller? Because I did not want to buy something over expensive now and it should actually work.

Sold and shipped by Amazon or third party?