If you add shipping as a metric from Shopify into a Custom Report in Polar Analytics, you may notice that you're not able to add product-related breakdowns to the report (such as Product Type, Product Title, or Product Tag).

This is because shipping is a metric defined at the order level, not at the product level, in Shopify. For example, a customers' shipping costs could have the same value regardless of whether they buy 10 products or 2 products. You'll notice that you do have the ability to break down shipping reports by order-related breakdowns (such as Order Id, Order Number, Order Tag, etc).