If you use the purchasing module as well as the parts module, you are able to add a purchase order when adding a part to a job.
To do so, open the job, tick the check box next to the part that you have ordered and click Change Status.
Select Ordered from the drop down menu
This will open a form allowing you to raise a purchase order.
When the invoice comes through, you will need to change this purchase order to an invoice in the usual way, through the purchasing tab.