Found this cute pack of cardstock at Michael's (half off! Didn't even have to use the standby 40& off coupon!):
Opened up a regular envelope the same size I wanted to make:
Traced the envelope onto the cardstock and cut:
Folded 'em up (I used a ruler to help make a sharper crease, didn't take a pic), and used double-sided tape to hold the flaps:
Ta-da! Created this stack in the time it took to watch "The Office" on the DVR!
I wrote the addresses on labels and then slapped them on - worked great! I may never use a "real" envelope again! : )