How do add class per item on an invoice

Invoice derive their classes from All Orders ship docs. By default the ship doc has one class for all the items. However you can enable a class per line by setting the appropriate preference. 

1. Close the ship doc 
2. Select Company --> preferences --> shipping 
3. Check 'Use classes on shipping documents' 
4. Click OK 
5. Reopen the Ship doc and select the class per line item.