It turns out the problem is caused by an unexpected sensitivity of the FedEx service to the order of sub-elements in the request's XML.
The request sent had:
But it seems FedEx expects ‘NumberOfCopies’ to be the first sub-element of ‘PrintOrderLineItem’ (before 'CatalogDocument').
After modifying the model of the ‘PrintOrderLineItem’ data structure to have ‘NumberOfCopies’ as its first element, the service worked correctly.
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