Just fixed this issue for a client. The plugin was giving that “Authorization failed” message. Just estimating based on order history it looks like the problem started sometime yesterday 2021-10-17.
Fixed it by replacing the default username at wp-admin > WooCommerce > Shipping > USPS with a new one created at https://www.usps.com/business/web-tools-apis/ . It looks like the default credentials that come with the plugin went invalid sometime in the last 48 hours.
