Setting up shipping

I am trying to work out how to resolve a shipping weight for a customer. We are using AusPost for shipping.

Their rates for this particular sized box is -

250 to 500 grams = $6.05

500 grams to 1000 grams = $12.60

1000 grams to 2000 grams = $24.80

The product we are working with can have two per box which is setup in the shipping properties for this particular product.

The carton for the product weighs 350 grams and the product weighs 300 grams.

If a customer bought one, it would weigh 300 + 350 = 650 grams and should cost $12.60 to ship.

If a customer bought two, it would weigh 300 + 300 + 350 = 950 grams and should cost $12.60 to ship.

There is no where that I can set the carton weight. I cannot put the product weight as 475 (Half the weight of two products in a box) because that would charge $6.05 for the product. I cannot put the weight over 500 grams because then it would charge the customer over 1000 grams and charge too much for shipping.

I have contacted CS Cart but they only want to offer me their development services.

Being able to add the carton weight to the shipping properties would resolve this. It would add the carton weight and the product weight can produce the correct weight for shipping.

Any suggestions on this one? Maybe I should add it to User Voice?