You have configured the plugin to use the Domain Masked url but you have not configured the domain masking option on 1ShoppingCart.com.

Using Domain Masking
If you would like to use domain masking, please do the following:

• Log into 1Shoppingcart.com.
• Mouseover the Setup tab and then select Domain Masking from the menu items.
• Follow the directions to implement the Domain Masking option.

Not Using Domain Masking
If you would prefer not to use Domain Masking, use the following as your Buy Now URL –

• Log into 1Shoppingcart.com.
• Mouseover the Setup tab and then select Site Links from the menu items.
• Copy the Add to Cart link up to the end of the {mid}. Be sure to exclude the ‘&pid={pid}’ at the end of the link.

Your link should look like -
http://www.mcssl.com/SecureCart/SecureCart.aspx?mid={Your mid}
- or -
http://www.1shoppingcart.com/SecureCart/SecureCart.aspx?mid={Your mid}

A snapshot of a 1ShoppingCart.com Site Links page is below -