Custom Shipping Calculator ?

[AzureCrystal]

Grateful thanks for any help or insights, I have a need for a special shipping calculator to work along ZenCart and I am not sure where I integrate this with ZenCart, it might be that I can use the Fedex(they use Fedex and I already installed the Fedex shipping module) module to do this ?? I am a good PHP/MySql/JavaScript coder, I am just not 100% sure where this would fit in ??

I attached a screen shot of the calculator they use, it is just the weight of each item times the quantity, summing the total, then they use that number when they call Fedex. I know, it might be something very stupidly simple I have overlooked, but I would REALLY appreciate anyone who can point me in the right direction, thanks !!! The individual mice shipping price would be easy to calculate from within the products edit section, but when they checkout I have to add the shipping price for the dry ice and the special cold box to the entire order, that is where I am lost, not sure how I can add those items to the final order !

In other words, using Zencart, what would be the simplest method of implementing that calculator, considering I already have the Fedex module ??