Services Tolerance for Max Ship Amount

I have defined a new Services Tolerance and tried to set a "Shipment Amount" value.  I clicked the checkbox and entered a value but when I saved and re-queried, the checkbox was not checked and the Ship Amount field was blank.  Is this a bug? Can I set a Shipment Amount tolerance for Services?  Or will Services invoices automatically go on Max Ship Amount hold if the invoice line total is greater than the PO Line total?   This is what appears to be happening in my system.  I thought that setting a Shipment Amount tolerance would allow me to validate