Here's another option...
You can buy cellophane of varying thicknesses by the roll. If done nicely, I've seen artwork wrapped in cellophane. (the two free ends folded into triangles and neatly taped on the back, so from the front it does look just like shrink wrap) It's much more durable than shrink wrap, without having anywhere near the same cost as individual sleeves or envelopes might.
This is actually what I do with my prints when I need to ship them (since I don't ship with glass). I wrap the mat, the print, and the backing board in an envelope of cellophane and put it back into the frame. That way it's still protected even before they put the glass in.
-=- Jen / Pixelscapes