this module adds a configurable fee for the Magento2 default cash on delivery payment Method
From Magento2 root dir:
composer require brandung/cash-on-delivery-fee
bin/magento module:enable Brandung_CashOnDeliveryFee
bin/magento setup:upgrade
also, if in production mode:
bin/magento setup:static-content:deploy
Go to Stores -> Configuration -> Sales -> Payment Methods If the Module has been installed successfully, there should be a new Configuration "Cash on Delivery Fee" under the Cash on Delivery Payment Method with a default value of 0.0
The Fee total will display in the Checkout and Order totals as soon as the Cash on Delivery Payment Method has been used and the value of the Fee is > 0.