You can embed a single product checkout onto your Unbounce landing page easily using the Chec product code snippet.
Click the share/embed button of your product and copy the embed checkout code snippet.
In your Unbounce page builder, drag the Custom HTML section onto your site and paste in the code snippet. Delete the "Buy Now</a> text from the snippet and click save.
Now create a button with the Box and Text features. Drag the box on top of the Custom HTML section you just created.
Style your button and text any way you want. You've now added a single product checkout to your Unbounce landing page!