Putting multiple Add to cart links / buttons on a page
jQuery is our friend.

Create a DIV for every product that will perform an ajax request (on click) to a path that will update your cart and then retrieve the cart contents. Ex. /ajax/cart?op=add&amp;pid=idOfProduct

Create a path (hook_menu) where you will send the add to cart message or other messages that you want to send to the cart (item removal for example).

This path will update the cart to your needs based on the parameters you send it.

The response of the path could be in full HTML for simplicity, with the updated and already rendered cart.

Replace the current cart with the new contents. It's not simple but it will work.

Hope that helps!