Step 1:

• Go to Integrations > Sales Channels

Step 2:

• Choose your eBay channel and click on "Open" button.

Step 3:

• Click Template & Policy -> Selling Format Template

Step 4:

• Click Edit on Selling format you want to add surcharge

Step 5:

• Select mode from the "Fixed Price Mode” dropdown.
• You can choose to add surcharge in dollar values ($) or in percentage (%)
• Set surcharge value
• Click Save

Step 6:

• Revise your existing eBay listing that use this Selling Format template.