Why isn't Shopify charging the extra shipping fee for additional items?
We charge you the store owner the product and shipping costs listed in the app. For shipping there is an additional cost listed lets say your customer orders 3 tee's the first one would ship $5.50(US order) then every additional tee would ship for $1.50 but if they were to buy a mug, tee and a phone case we would charge you full price shipping on each since they come off of different production lines and are typically shipped from different facilities. Unfortunately there isn't away to setup the additional shipping cost like we charge you the store owner if a customer buys multiple of the same product style. We recommend offering free shipping and including the shipping costs into your product costs.
Remember anything above our product and shipping costs would be your profit!