wenn überhaupt, dann thank-you discount

It sounds OK to me.

It is possible to write "thankyou" as one word, isn't it? I know that it is used by native speakers as well.

In my opinion, they are two separate words, and should be treated as such. :-)

I honestly do not know anyone who writes "thankyou" as one word. But maybe I've missed something.

@Anja: Even native speakers can misspell words, don't you think?

Yes, you are right. And, of course, in English these word combinations like in German are not often used (different words combined in one word). Thank you!

Looking for alternatives?

Here's one:

A gratitude discount.