So I'm super excited to announce that I am now able to confidently answer yes to that question. Daisy Design Shop has started offering RSVP postcards, as well as save-the-date postcards in any design! These postcards are made of a high-quality, slightly glossy, and extremely durable type of paper. Each set must be specially ordered, so they will take a little longer than my usual processing time (up to 3 weeks), but they're totally worth it!
Both the front and the back are completely customizable. Here are some examples of RSVP postcards:
Postcards can save on postage too! Here is a save-the-date design: