Auto Close Purchase Orders on Final Invoice check box on the Info tab of the PO Company Parameters form.

Select this check box to auto-close purchase orders when fully invoiced (current = invoiced, and remaining committed costs in Job Cost are zero). Exceptions for JC cost types that you want to be excluded from auto-closing can be specified on the PO Close JC Exceptions tab; exceptions for SM cost types that you want to be excluded from auto-closing can be specified on the PO Close SM Exceptions tab.