You can order usps boxes free on line..they have the regular priority boxes and the flat rate same boxes different spelling=much cheaper shipping rates.They deliver them right to you then if you mail using online shipping it is way cheaper with free DC than the po is.
They will even send an e-mail to the person letting them know the dc# You could use any plain box if you have some but the po is free so why not.I even get free tape from them(I am cheap) You do have to have accurate scales to weigh the boxes if your doing it all at home and not using the flat rate option. A dozen eggs packed with dc runs between $7-$8 most everyplace I have sent them.It is much higher though if you take them into the po to get stamped.
I keep up with all of my shipping with my computer programs we already have installed-it's our business computer so it only has boring record keeping stuff on it.