Add Donation Button to Shopping Cart Print
Full Answer
The first thing you need to do is create a product that will be your donation. This product will be a single-option, with a single variant worth $0.01.
Create a link list called “Donation”. Add a product link, select “Product” under Links To, and then select the “Donation” product. It should look like this:
This step is pretty theme-dependent. If you need a hand with placing and styling the textbox, please check with the designer of your theme or ask in the design forums.
Inside your Template Editor, create a new snippet called “donation-snippet”. Add the contents of this gist inside it.