A buy button is a shortcut to the checkout process. With the Shopify Buy Button, merchants can generate an embeddable product card and checkout button that can be added to landing pages or blog posts.
At the moment, Shopify doesn't allow us to modify checkout in any way (unless you're on Shopify Plus). They want to make sure the checkout is as secure as possible for everyone and that code changes to it won't 'break' the checkout process or put customers' personal information at risk.
Because of this limitation, it's not currently possible for us to place the widget in the checkout where Buy Button customers would see it.
Please contact Shopify support for any further questions about the Shopify Buy Button.