How to implement custom quantity selectors on product page using liquid?

Hey guys, I’m having a hard time trying to come up with a solution for a shopify implementation on the product page.

It looks simple, but I’m struggling with it.

Basically, this is a custom quantity selector. This store sells towals, so for the product page they’re offering bundles; in this case, if the user selects the box on the left, then the quantity of the cart should be updated to only one product, otherwise if the user selects the one on the right, then the quantity of the product should be increased to 2 and apply a 35% discount on the price of the second towal.

How would you guys go about it using liquid?