The quarter-inch postal rule.

[Greatgardens]

This has bitten me 4 times now (4 different sellers). Basically, if a small "bubble mailer" is thicker than 1/4", then it becomes a package and not an envelope. Each time, I get a "dun" from the USPS that I need to give them $2.50+ postage due. Big sellers seem to be quite aware of this, but smaller ones continue to send "thick" mailers. Each time, I do write the sellers and nicely explain the rules. I have not run across this twice from the same seller. Check it out at usps.com (calculate a price).