Adding a tip/donation to checkout page

Site URL: https://bezathreads.org/shop

Hi - 

We are a non-profit retailer funded by both donations and merchandise purchases. We have a donations page, and we have a shop for our products. 

We'd like to have an option to add a donation during the checkout process. Similar to adding a tip, etc. 

Is anyone familiar with how we can make this possible?

Thanks!

The checkout page can not be modified. It is a Squarespace security feature.

See this previous post