Could be, we use the USPS Shipping method. Perhaps somewhere in your settings is a User ID to access the USPS API. My Woocommerce installation had a default ID that worked for over a year with the USPS service until yesterday.
Under WooCommerce Setting, click the Shipping Tab, then you have 4 sub-tabs:
Shipping zones |Shipping options |Shipping classes |USPS |WooCommerce Shipping
Click on USPS and there should be a field for USPS User ID and a checkbox to turn on debug mode. The User ID you get from registering with the USPS link above goes into this field replacing the default Woocommerce ID.
Got it working, had to create an account at USPS, here’s where you can create that: https://www.usps.com/business/web-tools-apis/
Filled out the UPSP registration form and got an email back with a Username that I took and plugged into the WooCommerce> Shipping> USPS. Apparently the default wooCommce username is not working.
