Another endorsement for PirateShip. I've been using them for a couple of years and it's amazing. Free to use and it generates discounted postage (USPS and UPS) for everything.

Even lets you insure for whatever level you want.

Plus it allows you to generate postage for someone to send things TO you. As long as the person can give dimensions and weight, you generate the postage and send them the PDF to print and affix.