You my feel fine! One of my friends actually went to her brothers wedding 2 days after giving birth.
I was good ten days after, but I suggest talking to your friend. If she is a true friend she will understand that your RSVP is tentative upon you and baby's health.
It's actually a shorter reception than most and i know it is less expensive than most. I'm sure it will be fabulous but not pricey. Where I had my wedding, we didn't have to give the final count until a couple days before... so maybe I could update it. Should I also imagine a definite no for bringing the baby?
Hmmmmm i would probably RSVP as a no unless it was one of my very close friends or family.
If you really want to go just ask her if it's ok if you come only for a few hours, as they may be paying a bit per person.