Announcement reposted from Amazon United States:
If you’re part of the seller-fulfilled network, you can update product dimensions and weight for Amazon prepaid return labels through a new returns feature.
This feature will reduce carrier fee adjustments, based on incorrect dimensions, to your Amazon prepaid return labels for return shipments.
If you bought the original shipping label using Amazon’s Buy Shipping services, the dimensions from the original label will automatically be used for generating the return label.
To update the dimensions for your products, click SKU attributes in your Seller Central return settings.
For more information, go to Prepaid returns for seller-fulfilled orders.