Michelle,
It would be so much easier for you if you signed up at USPS.com and just printed out your postage. You just tape it to the boxes and turn in a package request pickup the day before. You have to turn in your package pickup the day before or no later than 2:00am of the day to pick them up. The mailman comes and picks up your packages for you. Of course if you live in an apartment this wouldn't work. If I did not have this feature I would think twice about selling stuff because the traffic to my post office is bad.
Dot
