I'm probably the last one to discover this but it turns out that Kobo has some books from
Amazon Publishing and they're discount eligible.
Kobo had a
book I was looking for and when I checked the publisher, I was surprised that it was Amazon. I thought "there's no way this is eligible", but tried anyway and was surprised that it was.
I'm not sure about any of the many Amazon imprints. I tried looking for some 47North titles, but didn't find them. Anyway, it may be worth checking with Kobo, especially if you have unused contest codes.