Advanced Shipping Price Rules - Shipping Cost on Shipping class subtotal

The ' Cost on Shipping class subtotal ' advanced shipping rules work with your flat rate shipping rules and settings. 

Using this, you can apply the shipping amount for a minimum and a maximum subtotal of items belonging to the added Product. 
And the amount or percentage entered in the ‘Fee Amount’ field will be used for increasing, decreasing, or using the same rate of shipping as per the category’s subtotal items in your cart.  
How it works:
Let’s say you have mentioned minimum Shipping class subtotal  ‘X’ and maximum Shipping class subtotal as ‘X2’. So, it has created a range of X1 to X2 Shipping class subtotal and we can add the X3 shipping charge for this range.
This advanced shipping amount will be added to the base price of the flat rate shipping amount, which you’ve filled in the Fee section of the main form (Shipping Fee configuration) of this method. 

Example: If "Heavy shipping" class product specific shipping charges.

Case 1: If 0.01 to 200 subtotal of "Heavy shipping" class products subtotal in the cart then it will add $5 
Case 2: If 201 to 299 subtotal of "Heavy shipping" class products subtotal in the cart then it will add $15 
Screenshot:

Advanced Shipping Price Rules - "Cost on Shipping class subtotal"

Cart page screenshot:

Cart page with shipping charges based on " Heavy shipping" class products subtotal.