If you wish to add an item (e.g. a delivery charge) to the sales order and not have this pull through on the purchase order, you add a manual line to the sales order that will go straight to invoice.
In order to add an invoice only line, you simply need to add a new line to the sales order however do not populate the code field, simply type in an invoice description I.E. 'Delivery' and change the pricing information before saving the line.