I just submitted mine because I'm lazy, don't forget to submit yours.
I also wrote in the email something to the effect that I never bought any gift cards with Apple Pay and I'm annoyed at Discover for making me waste my time submitting receipts and that it makes me want to use Discover less. All this is true.
Frankly, it is annoying and I want to avoid it in the future and maybe they will do something nice for us to make it up. Who knows.