Merchants on Shopify+ Plan can modify their Checkout page via theme editor and add Virtue Checkout Donations to it.
Please note, if your store is not on Shopify+ you are still able to offer Customer Donations before and after the checkout. However, at this point, Shopify only allows it's Shopify+ customers to edit the Checkout page.
The Checkout Page Widget enabled customers to add a donation at checkout.
Add the Virtue checkout donations widget to your checkout
Head to Shopify Admin > Online Store > Themes > Customize.
From the menu at the top of the theme editor, locate Checkout.*
From the Apps menu select Virtue customer donations.
Click save in the top right hand corner to see the change take effect.
Enable customer giving to see the widget appear on your store.
*Alternatively, the same page can be found by going to Shopify Admin > settings > Checkout and clicking on customise next to the checkout profile to which you wish to add the widget.