Great suggestions! Also, if you can't get the sandarac, you could try spraying the envelopes lightly with a hairspray like Aquanet. I used it once in a pinch. It made a slight texture to the paper but it worked. you can also make your own sizing from gelatin and water.
I had some envelopes once that wouldn't take any ink (they weren't sized at all). So I made the gelatin sizing and had to paint it on all the envelopes, let them dry, and then could letter on them. Pain but it worked.
The sandarac works great but I am allergic to it. After I pounced over a couple of envelopes and started lettering, my hand started itching like crazy.