Can White Christmas work on checkout pages?

Unfortunately, Shopify apps cannot run on checkout pages and for that reason, the app can't be displayed on the checkout page. 

This is a Shopify intentional policy; therefore, no app can modify that without breaking the Shopify security 'requirements' stated at https://help.shopify.com/en/api/app-store/listing-in-the-app-store/app-requirements#a-security

Please note that this applies to all Shopify apps, not only to White Christmas.