You can enable special instructions field (checkout note) inside the cart page.
Sticky Add to Cart button (on mobile only)
Inside the product page on mobile, you have the option to have the Add to cart bar sticky at the bottom of the mobile screen as the customer scrolls down.
By default, the Add to cart button is inside the product page content. This options allows it to float over the content and be sticky at the bottom of the screen.
Sticky Add to cart bar (on desktop only)