With the help of Extra Fee, you can create additional charges that can be both refundable or not, depending on your requirements. Let's take a look at that in detail.
To start with creating a fee, please go to Magento Admin Panel → Stores → Configuration → Amasty Extensions → Extra Fee → Fees and select Add New Fee. To determine a fee type, navigate to the General tab and locate the Eligible for Refund setting.
As for creating refunds, the behavior of fees will depend on the setting you've chosen. To issue a refund, please open an invoiced order and select the Credit Memo option.