Chuck,

I'm not sure that I follow the logic. I use ZenCart without any shipping mods. I assume that since ZenCart sends USPS PRIORITY MAIL as the shipmethod, that S.com logic is supposed to use USPS Priority instead of FirstClass. There seems to be a bug if that is the case.

Data requested by Stamps.com (1st few lines) Notice Ship Method
<ShipmentOrders>
−
<ShipmentOrder>
<OrderDate>2010-10-18T21:25:22Z</OrderDate>
<ItemsTotal>50.89</ItemsTotal>
<Total>58.01</Total>
<ShippingChargesPaid>7.12</ShippingChargesPaid>
−
<ShipMethod>
United States Postal Service (Priority Mail (2 - 3 days))
</ShipMethod>

Entered into Stamps.com application from the data downloaded
Postage cost $1.42
Mailpiece: Package
Mail Class: FIRST CLASS MAIL