Adjusting shipping rates

Hi.
We ship a lot of light-weight but large items.
The shipping manager under prices those orders.
The weight is small, but the boxes are extra large.
The post office now mostly charges by box size.
We are losing money because of this.
Can you help me customize the shipping manager to give a more accurate shipping price for certain items in my store?
Or suggest another (better/easier) remedy?
Thanks!
-Dhara

Would it solve your problem if AllProWebTools could detect a certain item in the customer's shopping cart and then NOT offer a specific shipping option?

Hi.
The shipping cost 'issue' occurs when a customer purchases 25 or more of our foam pads. This happens fairly regularly, since we are a whole-seller.
Shipping that many pads requires two big boxes and the shipping manager never offers the correct cost.
So, it's not the item that's the issue, but how many of that item they buy.