Hi Lexi,
Please make sure you have enabled the real time rates, and make sure your products have weight and dimensions in the product level. Also make sure you have entered the correct USPS Webtools ID in the plugin settings.
If you still face issue please provide the debug request and response, please remove the account details from the debug data.To get the debug data, turn on the debug mode in the plugin settings and while on the cart page copy the debug data and paste it here.
Regards,
Lorenzo
Team Xadapter