I'm evaluating switching to either Endicia or Stamps.com (from a fairly well tuned but somewhat manual system I use today). Would you believe I'd pretty much written off using Stamps.com, until one last google search turned up this thread?
I am only interested in a "hands off" XML interface, which I can fully automate from my own custom software.
Why not use Stamps.com? Well, I registered on your developer site to get the XML spec. I downloaded version 1.1, which is dated 11 Nov 2009 and is the only version offered. On page 3, it says:
Stamps.com will soon add additional ways to process an XML file using the client. These include the ability to request that a folder be monitored for XML files or have an XML file processed on demand.
Endicia's XML documentation describes running their Dazzle program from the command line with an option to specify the XML file. Comparing only the published documentation, using Endicia over Stamps.com seems like a no-brainer!
I hope you'll update your XML spec.