If you have a printed label with postage, the kind you can print off the USPS site, or PayPal site, you don't have to stand in line to mail it. Even if it's over 13 ounces, you can just drop it in the (wide) slot at the post office. I asked why this was so, and the answer was "we already have your identification from the online site where you printed this".

You also save about 20% on the cost of postage by using PayPal's service (which is free).

Might be especially useful at holiday time when the lines at the window are long.