You can add per package fees independantly per unit, or else add it to the cost dependancies via the shipping methods/settings.
1x ERR7171 = 300gms = $5 postage
- 3 Postage and Handling charge = $8
10x ERR7171 = 3kgs = $15 postage + $3per unit (30) = $45
These are extreme examples but are possible