You can set a custom fee to be automatically added on the customer's total bill on the checkout page. It shows up on the Taxes & Fees section, the fees are broken down when you hover the mouse pointer over the section.
Step 1 - Go Ecommerce Settings Dashboard & Create a New Custom Fee
Navigate to the bottom icon and hover over it. Select "Ecommerce Settings"
Open the "Custom Fees" Tab
Click the "Create New Custom Fee" button to begin creating new Fees as the selection will be empty at first.
Note: The list of newly created fees will show up under the selection bar. To edit existing fees you created, click on the pencil icon next to the created fees.
Step 2: Fill out the Custom Fee menu with the Name of the fee, a short description, the type of fee to apply at checkout, and the value in percent or in a fixed amount.
Created Custom Fees cannot be deleted so in the creation/editing menu, you can choose to activate or deactivate specific fees.