Hi,
Which is the shipping carrier you are using to ship the product, currently?
Have you installed any shipping app?
Where are you from?
When you’re fulfilling an order, you need to choose a package that matches the type that you want to use.
You can save your preferred package types on the shipping and delivery settings page or on an order’s Fulfillment page in your Shopify admin. Most carriers charge by size and weight, you can choose which one of your saved packages you want to use.
I would suggest the Multi-carrier shipping label app, which is integrated with multiple shipping carriers and the app automatically chooses the perfect box for the product based on weight and dimensions, so customers are only charged shipping charges for the box.