there are many action hooks in the order page you could use to add things (using shortcodes i would have thought).
as an example you could do – in your functions.php :

add_action('wppizza_gateway_choice_before', 'my_function');
function my_function(){
echo do_shortcode('[wppizza single="11"]');
}

or any other shortcodes you want to use

search templates/wppizza-orders.php for anything that says “do_action” for a variety of places where you could hook into

example updated

PS: this does not do any popup (if you want that sort of thing , you have to develop it yourself)
it just adds some items to the checkout page to choose from

(not tested by the way, what you do from here is up to you)