Mark, Luis did you contact the USPS Internet Customer Care Center (ICCC) to be granted permission to use the API?
X-Cart uses API=RateV4
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Your Web Tools User ID, shown above, is required to integrate with USPS Web Tools APIs. In order to start your integration process you must contact the USPS Internet Customer Care Center (ICCC) via phone or email to be granted permission to use the APIs you need. They will switch your profile to allow you access to the production server and will provide you with the production URLs.
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