Mine came with it, but I was really unsure if it was needed. And since there were no real instructions, I had assumed it was a vapor barrier. It made sense to me.
My guess is you could go without it. Otherwise, since the season is coming to a close, you could wait and reach out to them to find out for sure. I debated on putting it in, but thought it could not hurt. This may be even more important if your original cover had thin spots or small tears. Since you bought yours with a new cover, they may assume you will not have issues.
Good luck!
Post up photos along the way! People showed way more interest in this project than I could have imagined. I but Roger at Seat Concepts didn't believe it either.