You may find it helpful to know the general formula we use when calculating shipping charges for each buyer's order:
(Rate × (Total product weights + Container weights)) + Container costs + Handling surcharge
Rate part would be determined either by a custom "Rate per weight unit" shipping rule, or otherwise by the standard USPS or UPS rate(s) if you set up any rules for those.
Note that you can "game" the formula by applying non-literal figures for various settings, so the formula adds up the way you want. For simplicity, we've left out the matter of ordered Quantity, which of course affects the total product weight and could multiply certain Package Type costs/weights according to the Packing Capacity you configure for each product—e.g., if the product has a Packing Capacity of 2, and a buyer orders 3 of that item, we'd factor-in 2 of the Package Type you specified for that product.