First open Norton Internet Security. Then choose Settings at the top of the screen.
Next choose Firewall.
Then look at the tabs and choose Program Control.
You'll get a list of programs that are already penetrating the Firewall. It will tell you when you click on a program whether the Firewall is allowing one-way or two-way traffic and in which direction. Scroll down the window until you see the app.
For Stamps.com the app is called ipostage.exe. Check that this file has supports two-way communications through the firewall.