Hi, where on my invoice should I be putting my customers purchase order number so i can retrieve the invoice in the event of a query ?

Hello Gberrington-btin, 

Welcome to the Community page, 

If you click on +new>invoice>little cog wheel top right>click on add custom field>tick purchase order> name it purchase order>save then you will see the box appear for it top left hand side to then enter the information on.