Tira here, from Shopify Support.
Good question. Are you looking to charge $5 USD per product added to the cart?
If so, you can set-up a Shipping Profile on your store. Shipping Profiles allow you to set conditions on your shipping products so that you can charge shipping per product.
For this set-up, you can use Combined Rates with Shipping Profiles to add together the rates so that they reflect at checkout as the total shipping rate. Check out our help doc with examples of combining rates for products in different profiles, within a single shipping profile, and when only one location can fulfill the order: Combined rates at checkout.
Let me know if that helps!
Thanks for your feedback
I am setting up a store with a single product, and have two shipping methods, (1) free shipping to all the countries, and no issue here. (2) express shipping only to USA at a cost of 5 USD per piece. Now when someone from USA who is interested in express shipping adds two pieces of that single product, the shipping rates stays at 5 USD, while it is supposed to go 10 USD.
So my question is, in this case, how to link the number of pieces ordered to the shipping rates when 5 USD express shipping is concerned.
What you'll want to do here is the following:
I attached a screenshot to give you an idea of what it should look like :)