Ok for anyone that runs into this problem, this is what I did to fix it:
1. Backup the 3.90 DB
2. Run the update scripts in order from 3.90 > 4.0, then 4.0 > 4.1, then 4.1 > 4.2. Note that if you have any special modules/themes that reference database objects, you may need to run update scripts for those things, which you can usually get from the plugin/theme provider.
3. Then in a new directory, upload all the Nopcommerce 4.20 files.
4. Then copy the following files from the v3.90 site directory
5. Rename file setting.txt to .dataSettings.json and InstalledPlugins.txt to plugins.json
6. Switch your website to use the new files in the new 4.20 site directory using your web host's control panel.
7. Go to your sites homepage in your web browser and it should come up normally.
To fix the USPS plugin issue:
1. Go to the Nopcommerce admin panel and go to Configuration > Shipping > Shipping providers and click 'Configure' and COPY THE USERNAME AND PASSWORD and keep it in a safe place.
2. Then go to Configuration > Plugins > Local Plugins and UNINSTALL the USPS plugin. (you'll need to click the 'Restart site/services' button to complete the uninstall)
3. Then DELETE the USPS plugin on the Local Plugins page by clicking DELETE. (you'll need to click the 'Restart site/services' button to complete the delete)
4. Then go to Configuration > Plugins > All Plugins and Themes and search for 'USPS' and click search. Then click on the plugin and it will take you to a download page. Download the plugin zip file.
5. Then UNZIP THE PLUGIN and locate the 'nopCommerce 4.20' folder. Inside that folder is the 'Shipping.USPS' folder.
6. Zip up ONLY the 'Shipping.USPS' folder and all the files that it contains.
7. Use your web host's file manager (or use FTP) to upload the zip file to your websites 4.20 site folder to the /Plugins/ directory.
8. Then use your web hosts file manager to UNZIP the zip file to the /Plugins/ directory. (Note: If the "Shipping.USPS" folder already exists in the /Plugins/ directory, then you may have to delete it first before unzipping the zip file.)
9. Once the file is unzipped, the "Shipping.USPS" folder should appear in the /Plugins/ directory with all the other usps code files inside it.
10. Then go to your Nopcommerce admin panel and go to Configuration > Plugins > Local Plugins and INSTALL the USPS plugin. (you'll need to click the 'Restart site/services' button to complete the install)
11. Then once it's installed, then go to Configuration > Shipping > Shipping providers and click 'Configure' and ADD THE USERNAME AND PASSWORD that you saved earlier and click SAVE.
That should get the USPS plugin working normally again.