Non-Stock purchase order joined to Sales Order

 Hi to all.

Often, if I sell some items to my customers, I need to buy some servicse ( transportation, etc. ) to deliver my items. 

Then, I need to insert a Non-Stock purchase order that must be joined to original sales order, because if I send less or more items, the total cost of the service may be different from the original.

Many thanks for suggestions.

Rgd,

Pasq