What's a girl to do? I turned to a 90's fad, Tea Staining. I've tea stained a tablecloth to disguise a spill, and a chunk of lace now and then to give it vintage flair, but this was a first for paper. Still, the principals are the same and I definitely got the desired effect.
bright white ---
--- tea stained
To begin, pour about two quarts of water into a large pot and add 8 tea bags. Bring to a boil and then let cool. After about 20 minutes the tea should reach full potency. Stack several old towels beside the pot to aid in drying. The tea will stain the towels as well as the paper so choose wisely.
Cut envelopes to desired size and open envelope flap (for even staining).
Immerse envelope in tea, handling with care as the wet envelopes will tear easily.
Remove envelope and lay on towel, lay second towel on top and pat to absorb excess liquid.
Lay envelopes to dry on flat surface. Drying time will be several hours, I leave mine over night.
The paper will warp as it dries, but can be ironed flat. I prefer the "lumpy" affect. It adds to the shabby chic style I love!