1) Be sure you have the latest version of the module installed that includes the new USPS International ship rates. You can find this at: http://virtuemart.net/index.php?option=com_smf&Itemid=71&topic=21235.msg52351#msg52351

2) If you don't already have one, register for a WebTools ID on the USPS web site at http://www.usps.com/webtools/welcome.htm. You should get a confirmation email from icustomercare@usps.com with your User ID (or WebTools ID), password, and confirming that your production access had been turned on.

3) Be sure you've set up an address for your store under Store > Edit Store, so that UPS knows what address you're shipping from.

Enter all the necessary data in VirtueMart under Store > Shipping module list > USPS module, and also turn USPS on as a shipping option under Admin > Configuration > Shipping.

4) Make sure your products have weights entered.

5) Call the USPS Web Tools tech support number and ask them to move you from a test server to a live server.

6) If you still have problems, check to see if your web host has issues with proxy servers. My host was GoDaddy, and they did indeed have some issues with proxy servers. They emailed me a script to take care of this.

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For further information regarding this module, check this forum thread.