Hi Sir,
I have full understand your project , that each product has its own shipping rate If user are ordering more then 1 item then customer have to pay shipment for each product based on product weight ,
This kind of approach is very good and mostly used in alibaba store, I have place many order and each order i have to pay for product shipment ,
I am also good in shopify , I can give you better option which app you should used for your shopify store. mean while i also create shopify app ..
I am online now you can message me I will help you ...