Non-Stock "Service" Items Can't be added to Service Lines

Can someone else confirm that Non-stock "Service" Items can't be added to the Service Lines of a Service Order? 

We've created a "Service" type Item (not a 'Service Item') because the service technicians consuming the item (coolant in this case) have no method to track the quantity of consumption in the field, but they do want to charge a flat rate fee for performing such a service on a case-by-case basis.

If we can't use a Non-stock "Service" Item in this way does anyone have any suggestions for how to charge a flat fee on a case-by-case basis in service order? We considered Stock Items, but that won't work because we don't have information about quantity. We considered Resource, but the end-customers really doesn't want to see charges for labor, and they aren't being charged for labor anyways - they are being charged for the coolant (as a flat charge). We don't want to mess with trip charges since this is not based on location and is not a very consistent add-on charge.

Thanks for any help on this one!

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