How to add a drop down list at checkout

Hi there, 

I am building a small e-commerce and want to include a drop-down list at checkout where customers can select a charity to which I will donate a percentage of the proceedings. 

How can I implement this please?

This is not possible. The checkout page is locked in and cannot be modified except by Squarespace.

You can add a "custom form" to each item – not the most elegant solution, but it will do in a pinch!