First we need to work out the type of shipping that is required.
Weight based shipping is when shipping is based on the total weight of items in the cart. For example, product A weighs 2 grams, product B weighs 3 grams. Customer buys product A and product B. Therefore, total weight is 5 grams. Then based on the shipping rules set, this total weight would cost x amount in shipping to the customer.
Flat fee shipping is when shipping is based on a flat fee per product. For example, product A costs a flat fee of $5 shipping, product B costs a flat fee of $10 shipping. Therefore, if a customer bought both product A and product B, total shipping would be $5 + $10 = $15. The total shipping amount is each flat fee shipping amount combined.