Flutter checkout cart editing

I want to introduce a fee to my checkout, that shows up at check out.. The fee is called packing/shopping fee and is calculated as follows, at checkout a count is taken of the number of items in cart, if count of number of items in cart is between 1-3 then shopping fee is 500. If count of number of items in cart is between 4-7 items price is 1000...therefore there is an increment of 500 for every 3 item count increment.