Pirate Ship is our shipper of choice. They are pretty cheap and good. Compared to USPS, they do not lose boxes, and they offer two-day shipping at a reasonable price. We initially went with Stamps.com, but it was a big mistake. They are awful. They do not integrate at all with Mac computers and their pricing and service are really bad.
Founder - GrandBox
1%Stacklist Startups Are Using Pirate Ship
Pirate Ship offers easy batch shipping software.
Visit the website: http://www.pirateship.com/