Ignoring the gawd-awful USPS/Star Wars Promotion, I ran across these. Apparently, you can pay current first class rates with a guarantee that these will remain valid forever. Of course, with the art of the letter (sadly) dying in favor of texting "OMG I'm eatin the BEST cheezebrgr," and bills getting e-payed, it makes sense for them to cash in now. Pretty stamp though.