How to Configure USPS Shipping Module
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. (I use a fulfillment company to do our shipping, so I used their address.)
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.