Coupon code does not work in shop

Hello,

I received a 20% coupon code for the Stardock store after taking part in a survey.
The expiration date is March 31.

Now I wanted to buy some Galactic Civ. IV - products, but the coupon code does not work.
It happens nothing when I enter it.
It's a valid code, though, because when I enter some random words, it tells me the code does not exist.
When I enter the right code on the other hand, it happens just nothing, as I said.

I also tried 3 different browsers.

Can you help me with that?

Thanks in advance.

Best regards Felix