Redirect to checkout when confirming appointment

Using Marketpress and Appointments+, when a user confirms an appointment slot, typically it would show the Paypal form. Since I am using Marketpress (using Authorize.net for payment gateway), I want to be able to make it so when they click on confirm, instead of showing the paypal form, it automatically redirects to a link of my choosing (which will be the checkout page).

I have the code where that process is handled

if ( response.mp == 1 ) {
$(".mp_buy_form")
.find("[name='variation']").remove().end()
.append("<input type='hidden' name='variation' />")
;
$(".mp_buy_form input[name='variation']").val(response.variation);
$(".mp_buy_form").show();
}
else {
$(".appointments-paypal").show();
}

I would just comment out $(".appointments-paypal").show(); and replace with windows.location = ‘url of my choice’ but that would require messing with the core code which means my changes could be lost on update. Is there something I can add to my functions.php file to handle the same?