Don't forget . . .

. . . you can also get self-adhesive envelopes now, which do not require licking.

Even if you wish to use traditional stamps and envelopes, you can "lick" the stamp and flap with a wet sponge or a wet cloth. If you're really concerned, use bottled water - though I have yet to hear of the authorities analysing the residue from tap water to deduce the source.