This is something that would be something that a lot of people have thought of, but never really had the inclination, time or money to put the whole thing together. However, Flavorlope has and they are now selling these envelopes that have a fruit flavour gum to seal it shut! Sounds like a great idea, but they do work out a little bit on the expensive side if you are sending a lot of letters.
According to the Flavorlope website this why these envelopes are so cool and tasty;
Take a lick at this! We offer the tastiest envelopes money can buy, a sweet assortment of Flavored Envelopes found only at Flavorlopes. We are confident you’ll find an enticing alternative to the dreaded lick of an envelope, with one of our flavorful combinations!
Prices start at £6.50 or $9.99 for the multi flavour pack of ten envelopes.