[inebriate]

i would think that you will need to edit cart.php's add to cart function to loop through the products, change all the inputs/selects to arrays in buy_now.tpl, ie: <input type="hidden" name="productid"> to <input type="hidden" name="productid[]">, and edit the customer_options.tpl to make each option be displayed rather than have it selected as well as have another form text input thats named appropiately, ie: <input type="text" name="amount[]"> (and remove the one from buy_now.tpl)