Offset credit card fees in Donation block?

Site URL: https://www.friendsofravenswoodschool.org/

We're a 501c3. Most of our donors donate through the site with a credit card. We'd like to add the ability for donors to offset their credit card fees with each transaction. 

On other templates, this basically is a check box asking them if they would like to offset or cover the cost of credit card fees. If they check yes, at checkout there's an additional line that shows the calcuated cost (2.9% + $0.30) of the fee, and then the total amount. 

How do we add this to the Squarespace donation block? Code? A plug-in? 

Thanks!

This isn't possible on Squarespace. Donations are managed on the checkout page where no code or plugins are allowed.

You can make a feature request by contacting Squarespace Customer Care.