The video below explains the Cart page Customer giving widget and how to set it up. Prefer reading? Just scroll down.
The Cart page Customer giving widget allows your customers to add a donation to their cart from the cart page. It can be placed anywhere on your store. By default it appears under the checkout button of your cart page.
For information about how to set up the customer donation options go here.
Users with Shopify 2.0 themes can find out more about how to add the widget here.
Turn on the Cart page Customer giving widget
Locate and click on Widgets in the side navigation menu which will take you to the Widgets page.
At the top of the Widgets page you will see the Widgets navigation bar. Click on Cart Page.
Under the Cart page Customer giving widget description you will see a toggle switch. Click this toggle and then click on save at the bottom of the page to enable the widget.
The widget will automatically appear on your cart page under the checkout button.