The USPS just announced they're going to put out their first-ever line of Scratch & Sniff stamps on June 20th!
The 10 different stamps will all have pictures of popsicles, fruits, chocolate, and root beer, and come with the corresponding scent. And they're "forever" stamps, so they should just be the same cost as other stamps.
I've been holding onto the same book of stamps for about 4 years...I don't see myself needing anymore stamps in the near future, based solely on how long it's taken me to use my LAST book of stamps...but when I eventually need them, I'll look for these.