Brief
On 28th July, 2013 USPS released minor structural changes to their existing services. Some shipping methods were renamed and an estimate of delivery day is added in the name. You have to update the script and rename methods to enable USPS back on your site.
 
Solution
1. Download this archive:
http://www.viart.com/downloads/usps_v4.zip
 
2. Unzip archive and upload 'usps_v4.php' into the 'shipping' folder of your shop replacing existing file.
 
3. Login to admin panel and go to Settings > Orders > Shipping Methods > Shipping Methods (USPS). Change "Shipping Code" field in the following methods:
 to to to 
 

The patch was updated according to received feedback. If you already installed a first version of the patch please make sure to download a fresh one (either clear browser cache or use a different browser) and rename methods using regexp as provided in description.
 
Regards,
ViArt Team

USPS script was updated with new curl logic. If your site hangs up on checkout or doesn't show shipping methods at all this patch will help you:
http://www.viart.com/downloads/usps_v4.zip
 
Regards,
Vera