The "Processing Fee" is the cost charged per online transaction. The percentage is based on the order amount, and the flat dollar amount is based on the number of transactions. Learn more HERE.
Admins with "Full Access" or "Limited Access - Financials" permissions can set a "Processing Fee Default."
Setting a default will automatically select the organization's preference on whether the Organization or Customer (participant) will take on the fee for Registrations, Invoices, and Memberships.
To set the Processing Fee Default: