Hi!
we make some optimizations in our nopcommerce theme, and i don't found plugin / possibility to make "sticky" add to cart button in mobile/desktop.
How it's works?
Here they show in shopify how to do this.
https://www.youtube.com/watch?v=uxnbz3y_qbw
Someone maybe do something similiar for nopcommerce? for me better place to stick is right side of screen.