Make a mini art gallery on your envelopes with stamps. Few people realize that their unused stamps don't expire. You can continue using your old stamps, like coins, indefinitely. Add them in a cluster until you get the shipping amount you need.
Add this great tag to an envelope from Jackbear Stamps
Get your own letter writing station from Up Up Creative
Or send a quick (and soft!)postcard hello from The Quilted House
Catching the wave of a lesser known holiday like this can also be helpful to your shop sales. The magazine Country Living published a small blurb on letter writing ideas in their April issue and suggested that their readers search Etsy for supplies. I have been promoting National Letter Writing Month in my Etsy shop announcements and tags for just two weeks and in that time three of the five items I specifically posted in honor of letters have sold. What a great way to get a bigger audience searching for a your items!