what procedure should we be using to correct the qty of shipped items on a posted invoice when the sales order is still open:  ex:  created a sales order for 50 widgets we shipped and invoiced for 20 leaving a backorder to fill of 30.  customer only actually receives 18.  
 
1.  a credit needs to be created for the customer of 2 widgets and the backorder needs to be increased from 30 to 32.
 
2.  currently we are going thru the release sales order and keying in a negative amount of (2) items to account for the shortage-which then will create an credit memo upon invoicing and increase our oustanding backorder.
 
is there a more effecient way of handling these types of issues.