When you're fulfilling an order using Shopify Shipping, 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 settings page or on an order's Fulfillment page in your Shopify admin. When you're ready to buy a shipping label for your shipment, you can choose which one of your saved packages you want to use.
Supported package types
You can use the following package types to ship your products with Shopify Shipping labels:
A regular package. The total of the length, width, and height can't be above 84 inches.
A padded envelope, bubble mailer, or plastic (Tyvek or Poly) sleeve. Packages thicker than 3/4 inches don't qualify for USPS First Class Mail rates.
A large, flat envelope no thicker than 3/4 inches.
Flat-rate packaging is supplied by your carrier. Its shipping cost is not determined by the weight of your shipment.
Unsupported package types
You can't use the following package types to ship your products with Shopify Shipping labels:
A flat envelope or letter no thicker than 1/4 inch.
A package whose total dimensions (to total of the package's length, width, and height) are greater than 84 inches.
A non-rectangular package.