If you ship your items using particular box sizes other than those included in Etsy's default box sizes, you have the option to add those under the Package Preferences section on the shipping settings page.
Visit Shop Manager > Settings > Shipping settings and select the Package preferences tab. Then click Add a package preference.
For sellers in the US, select your package type. If you choose Custom, you’ll be able to enter specific package dimensions and a name for the package type. You may also set a package-level handling fee that will be applied to the order, regardless of the number of items. The package fee will be applied in addition to any handling fees that you set as part of your calculated shipping profiles.
You’ll also have the option to select from a list of Flat Rate or Regional Rate package types. If you use Flat Rate or Regional Rate packages and a buyer selects a mail class that is compatible, those rates will automatically be calculated as long as the order is within the Flat Rate or Regional Rate weight and dimension limits.
Note: Make sure that you offer mail classes that support Flat Rates and or Regional Rates like Priority Mail, Priority Mail Express, and their international equivalents.
For sellers in Canada, you can set up a custom package preference and name it whatever you wish. Package fees are not available.
When you are done, click Save.
Once you add your Package Preferences you will have the option whether or not to use common Etsy package sizes in addition to the preferences that you set.
To see a list of the packages and their dimensions click the ? next to Use common Etsy package sizes.
We will automatically calculate the smallest and cheapest package to use for each order based on the selected mail class and the total weight and dimensions for the order.
See how to set up calculated shipping profiles and add weights and dimensions to your listings in the Calculated Shipping Help article.
Here's a quick video on how to get started using Calculated Shipping on Etsy: