After some further investigation, the problem seems to be that the INI setting "Handler" under the "[AccountSettings]" section in shopaccount.ini somehow has presedence over the shopaccounthandler specified in the ezorder table when eZ Publish displays an order. This seems like a bug to me, but if it is, it seems strange that no one has reported it before (at least I can't find any information about it in the bug reports).
As it turns out, this actally is a bug. At least IMHO. When displaying an order, eZP uses the shopaccounthandler specified in the INI setting instead of the one specified in the ezorder table.