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Can you please provide us more details about USPS API?
We currently do not have existing integration using this API, but if you can provide us more details about it, we can also check for possible workaround or other option.
I also suggest checking this user guide: http://www.jotform.com/help/188-How-to-add-Shipping-to-your-Paypal-standard-account
Perhaps the existing shipping option will work for your requirements already.