Im sure there are tons of related queastions going int he same direction – i think cluse.com does a good job in bundling products (offering additional products on a product page)
Can anyone tell me which app they are using?
What I really like is that the offered products can be added without the main product on the page.
It looks like it could be rendered by liquid so maybe it's a custom app development and the theme has been modified?
App Selly can help you to display recommended products at the bottom of page:
We can also move the recommendation section to below the add to cart button for your store.
Actually you can do this without an app.
We are doing the same thing for a client to to create colour swatches in a group but it would work for this in the same way too.
I can set this up for you in about 2-3 hours. Contact us at firstname.lastname@example.org if you want to progress with it.