My fiance and I were struggling a little with how to do our RSVPs. We wanted to do them online, but we knew it was more traditional to do the cards with the postage on them. We went back and forth for a while and decided to do both because the RSVPs came with our invitation anyways, but because we included an online RSVP option we opted to NOT include postage for the return RSVP.
Our only concern was his grandma. He said she wasn't very tech savvy as far as he knew and he didn't want her to have to go out and buy her own stamp either. I said we could put a stamp on just hers, but instead he called his mom and asked that she please help grandma RSVP online so she didn't need to worry about stamps or have a fight with the computer.
We sent out the invitations and... grandma was our very first RSVP. She did it online with no help from his mom.
We got the email that she confirmed her attendance a 6:45 a.m. the day after we sent the invites. I saw it at work and sent my fiance a message because I was cracking up and he responded "seriously? I didn't even know grandma had a computer...whoops." After that he called his mom and let her know that she didn't need to help grandma after all. She asked "why, did you include the return postage after all?" And he answered "Nope. Grandma already got her invitation and responded!"
Since then most of our guests have opted to RSVP online. We've only gotten two paper cards back. One from a relative who thought it make a nice keepsake for us and one from his uncle who said "grandma said she's an old bat and SHE figured it out and if I can't then ought to pay for postage."