We highly recommend that you apply for a Business Reply Mail permit from the U.S.P.S. Your response rates will improve because not everybody will use your toll free 800 number. It takes approximately three (3) days to get the permit number and your zip + four for the RSVP card. By having this permit, respondents can mail back the RSVP card without having to pay postage. Without the permit, they will have to apply a first class stamp and it will affect your response.
You will need to set up your account by using Form3615. Once you have filled out the application you will need to pay the permit fee ($160) and any additional fee that the post office requires. Click here for a complete outline of postal requirements to utilize this feature for your mailings.