I like Shippo for several reasons. Every other shipping app I looked at, my business was between price levels: we ship too much to get the free version, but don't ship nearly enough to justify the monthly cost for the paid version. Shippo is cheap and the cost is per label, which is perfect for my business.
After getting started with Shippo, I needed support a couple of times, and they were super fast to respond! I really didn't expect that, and it's so nice to do business with people who have great customer service.