You can add a "PayPal Donate" button to your website. You may have to edit the ".liquid" template files of the theme you are using. Here is a link where you can create a donate button.
Hey @rich47 , welcome to the forums!
Unless you are a Shopify Plus member, you cannot edit the Checkout page.
I wrote an app that does something very similar, check out out here: https://apps.shopify.com/donatemate Drop me an email and we can have a look at customising it for your needs!
Hey Rich - Shopify enabled a "Tip" feature in the store. We actually configured this to allow people to "Donate" rather than "tip" at checkout. It's a pretty great tool, I just need to better understand how we track the tips! We have a charitable giving to Local Laundry, so giving customers the option to contribute to our goal of raising $1M for local charities was pretty key. There was some form editing, but it took no time at all. Hope that helps! Best of luck!