http://www.morfon.com/printing-envelopes-inkjet/

How to maintain the inkjet print run when wet?
Does anyone know how to keep anything printed on a printer Inkjet run when wet? An aerosol, perhaps? Is discomfort that smears so easily, especially on the envelopes.
You could try a clear acrylic spray, like artists used to keep his work spots. Suggestion, Walmart Krylon works like the fancy stuff from Dick Blick source art. If you really want to keep something forever, it laminated.
Epson Stylus NX115 Color Inkjet All-in-One Printing envelopes