The easiest thing would be to deactivate that service item and create a new non-inventory item. You may be running into trouble because you are trying to change a default QB item. You can map the new non-inventory account to the same income account as the old service item. Just be sure to rename the service item something like "xService Fee" when you make in inactive so that your new non-inventory item can be named "Service Fee" without a problem.
I have used mHelpDesk with QB. One thing to watch out for is making sure all the payments and invoices import. I regularly ran into issues with invoices not importing and it was usually a service or inventory item that got created in mHelpDesk that wasn't already set up in QB. If the invoice still won't import after adding the new service/inventory item, then you may have to manually enter it in QB. For the majority of invoices, the import worked just fine. The problem happened when my client would make new items in mHelpDesk.
