Wedding Invitation Dilemma

So we are having an huge wedding invitation dilemma!! We wanted to create invitations that were more than just the ordinary traditional ones. We met with a independent business owner of “Send out Cards.” The great thing about this company is that they allow you to create your cards online. We can add our style font, pictures and other clip art items to the invitations. Once we have created the card we are able to send them to the company who will print them out, stuff them, stamp them, and send them to our guest list!! The cards will be delivered 48 hours after we have requested them! An amazing turn around. Here is our issue. We are not able to put an extra stamp on the return RSVP postcard that will come back to us because it is physically not possible with how their system works with packaging the cards. So we would insert text that would say place stamp here. Now we understand that the best etiquette is to make sure that the stamp is on there, but its not possible for this instance. Some thoughts we have include; most everyone has a stamp lying around and it wont be a problem. Other thoughts include; If they really want to attend the wedding they will find a stamp and deliver it in the mail in a timely manner. Final thoughts include; people will ridicule, judge, blame(if they dont get it to us in time), and be offended…thoughts???