I'm glad you found it, sorry I didn't get back to you earlier.
You can also, if you like, add after custom filed name in the same box the total price with the currency symbol but it will beshown in the cart as well!
So your custom field can show the total price instead of wrong taxed additional fee. You can also write custom field fee of course there. Whatever you like but remember it will be text only and you will need to edit it when you change the custom field value as well.