commerce_shipping-8.x-2.0-rc2/commerce_shipping.commerce_package_types.yml

custom_box:
  remote_id: custom
  label: 'Custom box (1x1x1 mm)'
  dimensions:
    length: 1
    width: 1
    height: 1
    unit: 'mm'

commerce_package_type:
  class: \Drupal\commerce_shipping\Plugin\Commerce\PackageType\PackageType
  deriver: \Drupal\commerce_shipping\Plugin\Deriver\PackageTypeDeriver