eConnect Error 110 when Updating Existing SOP Order: Item number/location code does not exist in inventory – Apr 10, Dynamics GP Land

This is a pretty obscure eConnect issue that had me puzzled for several minutes.

I'm working on an upgrade of a client's existing eConnect integration.  One component of the integration involves updating existing SOP Orders in Dynamics GP.

When the integration code submits the order update to eConnect on my development server, I get this error...

Continued on Source: Dynamics GP Land: eConnect Error 110 when Updating Existing SOP Order: Item number/location code does not exist in inventory