I installed the Stamps.com Connect to my computer and I can't uninstall
Your application is probably running in background, so you have to "kill it" before you can uninstall it.

Using Activity Monitor, find the Stamps.com Connect process and double click on it (using Acrobat as an example because I don't have Stamps.com; the process is the same, however).



You will then be presented with another dialog which will allow you to quit the running process:



Select "Quit."

You should then be able to uninstall the software.  As for uninstalling it, I recommend using AppCleaner.  I have used it for many years and it has been successful in removing even the most stubborn of Apps.  Plus, it's free.