Hi there! I see you're having trouble with the USPS app on your Mac running OS X 10.15.7. Could you tell me a bit more about what specifically isn’t working? For example, is the app not opening at all, freezing, not tracking packages, or something else? That will help me guide you better.

While I wait for your details, here are some basic steps we can try to get started:

1. Restart Your Mac: Sometimes a simple restart clears up underlying issues affecting apps.
2. Check for App Updates: Confirm you have the latest version of the USPS app installed from the App Store. Older versions might have problems.
3. Reinstall the USPS App: Uninstall the current app and then reinstall it. This clears corrupted data or files that could be causing trouble.
4. Check Your Internet Connection: Make sure your Mac is connected to a stable network. Sometimes slow or spotty internet affects app functionality.
5. Clear Your Browser Cache (if you are accessing USPS via browser instead of app): For Safari on your Mac, go to Safari > Preferences > Privacy > Manage Website Data > Remove all.

Let me know which of these you've tried and if you see any error messages or specific behaviors from the app when you try to use it. That way, I can help you with the next steps!