This was one of the issues we struggled with as well. Ultimately we wrote our own UPS, FedEx, and USPS integrations in-house, but I can say that Shippo was incredibly responsive to our questions and requests, even going so far as to add a few of the features that were imperative to our business (and I'm sure many others.)

I still think Shippo has a long way to go, but they're doing great work and are very responsive to new ideas.