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The client application credentials provided in the request are missing, invalid, inactive or not approved for access
While being connected to the REST API. Please mind that USPS has two new portals through which you can go onboarding, both look identical, they differ with “developerS” / “developer”:
New – with plural Developers: https://developerS.usps.com/getting-started (We and our users are interested in this one).
Old – with singular Developer: https://developer.usps.com/getting-started (This one positions first in Google results, but we are not interested in it).
Please ensure that your REST API credentials are created with the first link. Then use them in our plugin. After completing these steps, please verify if the issue has been resolved.