Great topic, I'll bring it up.
pirateship and also sendle are my favorite services to save on usps as they are the cheapest and fastest for usps in the usa.
I can also recommend a service for buying usps shipping labels - labels.vc (all functions open after registration), especially good for wholesalers and help to save money.
Also some people recommend parcelmonkey, but as for me this service is very inferior to the first two.