Confused to which shipping method to use

Hi
I have products of varies weights and the products will be delivered by palette if the order is large enough. The more someone buys the cheaper the shipping cost which we want to pass on.
Shipping costs
1-20Kilo 9.00
21-30Kilo 14.00
31-40Kilo 18.00
41-49kilo 22.00
50Kilo+ 35.00
I have tried the table method but goes wrong when the overall weight exceeds a certain level. i.e. If a product is 31Kilo and you order 3 the delivery charge should be 18 X 3 =54 but you actually get a larger shipping cost. Is there something I can change to fix this?
Thanks for any help

This may be as simple as just increasing your Admin > Configuration > Shipping > "Enter the Maximum Package Weight you will ship" value as when you go over this level (default 50) a different calculation method kicks in.

Thanks for the reply but when I do that if you order 2 of the same product it shows zero delivery.