We just started using Shippo for our shipping needs both on and off our ecommerce site. Their app looks great and their documentation/API is among the best I've worked with, so the integration with our website was a lot easier than expected.
The software has a few quirks sometimes, but nothing major, and they well make up for it with a really responsive and great customer support! I would definitely recommend others to give Shippo a try...we previously used Stamps.com that we felt was a bit limited. Overall a great experience!