Would it be logical for the following idea:
Categorise the products into different delivery tiers, the very large items (which would in reality require a longer delivery time) would be exempt from delivery calculation as this item would be sent seperatly (the client could add on extra onto that product price to compensate that it would be added to the final delivery cost) then my calculations would be easier on the smaller items where I could calculate the weight and size and determine the delivery cost on those only?
Chris