We have base rate which is always added with the addition cost of the shipping.

Example: Base rate + surcharge shipping

Now if you want to revise the base rate based on condition then you can do it using Base Rate Price Tier.

You can modify the base rate using Price, Weight, QTY, and Distance.

Select the condition based on your need and hit the Add Tier.

Here you can set the Min cart Price and Max cart price then the Base price will be.

Here you can see in above screenshot my default base price is $10 but if cart total is in between $100 to $200 then the base price will be $5.

With base rate tire if you want to add some addition rate like base on QTY, Base on cart Percentage, And base on weight then here is the way.

Here in this image I set the addons price like 9 per item, 10% of my cart total, and 9 per KG.

Example:

Here how the final shipping rate is count by the app. My cart total is $150, Total 3 QTY and 2KG weight.

My cart total is $150 so the base price is $5.

Base price $5 + 9*3 ($9 per item rate) + $15 (150*10%) + 9*2 (each kg weight charge $9) = $65 final shipping rate for my Shopify checkout.

I hope you understand this calculation about this additional rate.