Hello.
Please, check the USPS version you are using. There were performed some changes by USPS several months ago so we have created a new module USPS V3. In case you use the old one, please follow the below listed instructions to add the new shipping module to your installed ViArt version:
1. Download archive
http://www.viart.com/downloads/va_add_usps_v3.zip;
2. Extract file usps_v3.php into the folder 'shipping' of your shop;
3. Extract file va_add_usps_v3.php into the root folder of your shop;
4. Run script
http://your_site.com/va_add_usps_v3.php in Internet browser;
5. Delete file va_add_usps_v3.php from the root folder of the shop.
6. Enter Admin panel, go to Administration > Shipping Modules and check if it appeared in the listing under name USPS V3.
7. After installing this shipping module you can set it up with the same settings as in the old USPS module.
All releases which were issued after May 2007 already include this new USPS shipping module.
With regards,
ViArt Support Team