My sister-in-law's wedding is on 3/19, and the poor girl has only received 7 RSVP's.  Now I'm contemplating not even stamping my RSVP's when it comes time to do it. You waste 44 cents per couple, and no one bothers to send it back. Next year, I think stamps will be 45 cents. We have 96 couples right now. That's $43.20 we could save. 

Seriously, what are they going to do with an envelope that has a stamp on it and is already addressed to you? Those people had better write you a letter in the near future!  

Here are my thoughts.  I probably won't put a stamp on the RSVP envelope, and I'll word the RSVP section like this: "Kindly RSVP by June 25 using the card provided or by logging on to: theknot.com/ourwedding/ournames"