I use the URL feature to integrate Endicia into a custom web-based shipping tool I wrote.
My main wish is that they would have a good API that I could call to better support round tripping information. I can easily get information about a shipment into Endicia, click the button to print the label and autogenerate the email, but getting the tracking number and other shipping info back out (so I can record it locally and also updated eBay) is a pain. I have a rickety system set up now using a text file and an AppleScript but it is fragile.
If you want to use a Mac as your shipping computer, I think Endicia is your option. If you're Windows based, then either is likely a good option. No love for Linux yet...

