We moved our shop from etsy and we have an enterprise account with canada post. We do all measuring, weighing and shipping documentation in house. Etsy allowed us to have a base fee for shipping to the US for Box A which we set from experience< which we know is 32$ lets say. When a customer orders 2 Items it goes in Box B which costs 8$ more. so the price the customer pays is 40$: add another item and the cost is now 48$ and goes in box C.
Everything is weight and volume calculation with canada post.
How do we do this with shopify??