I would use TAS-R and
 
select * from bkarinv where bkar_inv_cuscod = 'XXX' and bkar_inv_cusord = 'YYY'
 
(XXX is the customer code and YYY is the customer PO)
 
And export it to Excel.  You can limit the fields rather than * if you want and then you can total in Excel

We don't have salesmen but I have just discovered SA-I allows me to print every shipment/invoice  to a customer for one single Purchase Order.
 
Someone may ask what have we shipped and invoiced off a certain customer PO.   Until now we had never found a good report that would do.  SA-I Print sales person summary is likity split as long as you enter ship date range.