I did a search and couldn't find a solution for this. When I take my products (of various size and shape) to USPS, they always charge me for shipping based on the size AND weight of the box I'm shipping my products in. I only see an option for weight on the product page and I only see some options for default boxes on the Shopify Shipping settings page. Is there a way to just provide a unique size and weight for each product and have the shipping cost calculated based on that?
I recently asked this question to support and I was suggested that if I wanted to give each product a box type that I would need an app such as Boxify to do this.
From what I gathered from the Calculated Rates shipping page. As long as all your items have weight and place a box size as a default it will auto calculated that for your customers at check out. If you don't use an app like the above.
Yes weight and size are essential particularly if you are using custom boxes. Shopify calculates the rates based on weight as majority of customers like it simple and want to use it that way. However, there are cases like yours where without dimensions, the rates won't be accurate and hence it is a must. You can use the Multi Carrier Shipping label app that allows you to set up product dimensions and then define boxes. Then based on the product's dimensions, the app decides which boxes to use for that order. This way the rates are also calculated accurately.
Was your problem solved yet? If not, to solve that problem, you can add USPS flat rate packaging on your Shopify store. In case you do not know how to do it, here is a simple guide on the topic of how to add USPS flat rate packaging on Shopify
By the way, if you have any trouble with shipping, you can visit https://avada.io/shopify/docs/shipping/ to find a suitable solution.
Hope it helps!