In the process of recharacterizing our IRA contributions (another badassity moment that is saving us $2500 on taxes), I realized that we were out of standard envelopes. I was fretting about when I would get to the store when I realized that I didn't have to send my forms in with a classic No 9 envelope. The post office will just as well deliver my forms in a leftover greeting card envelope from my wedding invitations (when we misprinted the date, whoops!). I've kept these envelopes with my greeting cards for 5 years (bad declutterer), and I just realized that they work just fine for sending regular mail. Plus now everyone gets pretty green envelopes! I'm not buying envelopes for a while.
