Shipping Costs by weight

In the Netherlands we have a postal system that defines the shipping costs per weight range. As in most countries I presume.

But in this particular case I have to add extra costs for products above a certain size.

What happens now in Marketpress is that in the checkout it adds up all the costs of the individual products plus the extra costs. So I end up with a ridiculous high amount of shipping costs.

I want it to rate it for the total weight and only add the extra costs for the large products.

How can I get Marketpress to do so?

Regards,
Saskia