https://woocommerce.com/feature-request/add-a-price-adjustment-field-for-boxes-on-the-fedex-shipping-method-extension/
Add a price adjustment field for boxes on the FedEx Shipping Method extension - WooCommerce
Add a price adjustment field at the box level for the FedEx Shipping Method extension. I currently ship products in various boxes, some are inexpensive ($0.75)...