Hey,
If you are referring to the Wix Stores Manual Payments, you can only add a disclaimer regarding the additional fee. It can be done through the site’s dashboard → settings → accept payments. Please note that non-code related issues are handled by Wix Support .
Also check these posts that can be helpful and if this information won’t help, specify what are your needs exactly so we can further look into it.
https://www.wix.com/corvid/forum/community-discussion/tax-and-additional-fees-1
https://www.wix.com/corvid/forum/community-discussion/how-to-calculate-a-custom-price-based-on-user-input