Free shipping can be done a couple of ways on Total >= 50.00 ...
In the Modules ... Order Totals ... Shipping ot_shipping where you set the Free Shipping on Orders >= 50.00 ...
Or, as an Option shipping method in the Modules ... Shipping ... Free Shipping Options freeoptions shipping module where you set this for Total >= 50.00 ...
The Free Shipping based on the Order Total module is a little harder to control to customize this as there is the Order Total module itself and then similar code is used else where in the code in a couple of places with the same logic being used ...
You might consider writting an Order Total module specifically for Hazardous Materials that regardless of the shipping could be used to add on the additional charges you need ...
You do need some method to identity these Hazardous Materials and decide is this a per item quantity or just a Hazard Fee in general ... or a combination ...
Or take it further and customize an Order Total module for Hazardous Materials based on the selected shipping module as the fees may vary ...
Sound like fun? :cool: