Related Questions:
Does stripe have a function to add on a processing fee?
Does ET have a way to charge a cc surcharge?
Can I pass Stripe fees onto a Customer?
How would I charge a one-off fee?
Answer:
Event Temple does not process any payments so this is not an existing feature on Invoices. You can manually charge this fee by adding a Menu Item for the amount you wish to charge.
If you are using Stripe, this question would be best directed to their team to confirm whether it's possible to implement from your Stripe account.