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API Authorization failure. User is not authorized to use API RateV4
To resolve this, please ensure that the credentials are correctly entered in the plugin’s API settings. If the credentials are correct, and you’re still experiencing the issue, you will need to contact USPS support for further assistance.
When contacting them, explain the authorization issue, so they can help you with the necessary authorization for the RateV4 API.
Additionally, the error might occur due to using a deprecated Web Tools API. In this case, we recommend switching to REST API, for which we have developed a documentation.