Very nice postcard. The concept is cool. The font is not so excellent though. But really, everything looks great. Maybe you could try redesigning the font. The rest, the format, colors, theme and feel should stay.
Can you try incorporating the yellow to the starfish? I think that would be great. From my perspective, with the tan starfish alone, the design looks dry. Yellow, I guess, will bring that to life.
The postcard idea is really nice. Good job on the design! I'm sure your guests will want to come after seeing these. And the pre-RSVP card is a smart, practical way to save money on expensive invites. A lot of people decide to make their own or maybe if you have friends who can help you out, that would be great too. Glad the actual prints turned out well.