The envelope size, fool. Don't stalk me, FBI.
ULINE Shipping Supplies has a distribution center just north of Chicago, actually fairly close to where I used to live. Not that it actually matters at all because shipping still costs dollars. For curiosity, the box of 100 smaller (9.75x12.25in) white self sealing envelopes cost me 39 bucks, while 100 brown (13x18in) tab sealing envelopes cost 70 dollaronis.
SPEAKING OF SHIPPING THOUGH: Here's a sick deal. Buying in bulk means having my own envelopes, which means no more buying individual envelopes from the post office, which means no more trips to the post office.