The underlying connection was closed
OR
The operation has timed-out
OR
Unable to connect to the remote server

If you receive this error, while PDshop is making an "HttpWebRequest" (see below), this usually indicates a server or network problem.  

The following PDshop features use Microsoft .Net's "HttpWebRequest" functions:

UPS, USPS, and InterShipper Rates
PayPal IPN
PayPal Direct & Express Checkout processing
Google Checkout processing
Authorize.net Processing
PDshop.Net Activation

The above PDshop features require some server to server communication.  In other words, during checkout PDshop will communicate with another server (to get the shipping rates, process the credit card payment, etc).  

If your server (or network) is misconfigured or asp.net not up-to-date, and you encounter errors or timeout messages while any of the features mentioned are executing, you should contact your web host (or server administrator) for assistance.  

*** One common cause for this error is an improperly configured firewall, proxy server, or other network equipment.  Contact your web host for a solution.

Also see the Microsoft Article listed below for some troubleshooting tips.

Article Applies to: ASP.NET (Microsoft's .NET), PDshop .NET