you’d have to edit the wppizza-cart.php template

huh?
it does that by default

in short, what you want to do is:
turn off all plugins. keep theme.
can you use quickedit ?
if not, switch theme to a default theme. can you use quickedit now ?
if you can, speak to theme developers to fix it

thanks:
your problem is javascript errors that are either thrown buy your plugin or theme

Uncaught Error: Syntax error, unrecognized expression: a[rel=]
Uncaught TypeError: Cannot read property 'hasClass' of undefined

you should de-activate all other plugins and use a default theme to track it down where those errors come from (as they are all thrown in together in a load.scripts.php i cannot tell you which one is the one likely to cause the problem)

that said, my initial suspicion would be your “jquery-updater” . i cannot even begin to tell you why it is a very bad idea to replace the jquery version that comes with wordpress that way
(but here’s a great article i fully agree with https://pippinsplugins.com/why-loading-your-own-jquery-is-irresponsible/ )
It may be the case that that plugin is not to blame , but de-activating it might at least make debugging the underlying issue easier

you should also enable debug like so https://support.wp-pizza.com/topic/how-to-enable-debug-in-wordpress/ and check your debug log

got a link ?

what version did you upgrade from ?

>phpmailer: Could not instantiate mail function….

that is a server or setup error. you need to enable debug and check your error logs
there are many many possible reasons for that error, none of which have anything to do with the plugin

a simple https://www.google.co.uk/#q=phpmailer:+Could+not+instantiate+mail+function will give you hundreds of results and possible solutions. Which is the right one for your setup I cannot tell you

also have a look here
https://support.wp-pizza.com/topic/order-does-not-send-email/

>Why is the phpmailer still trying to run when I’ve chosen to use SMTP for sending emails?
phpmailer will ALWAYS run, you just use smtp for authentication. one doesn’t really have anything to do with the other

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)

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