I don't seem to have access to callService in my patch

Nevermind I managed to make it work, thanks for your answer

Have you tried this ?
import { CartNotification } from "@website_sale/js/notification/cart_notification/cart_notification";

Unless I'm mistaken, since I'm patching the legacy AddToCart widget I cannot use Owl components or services to create the notification?

Yes, but could you tell me what you're trying to customize? Maybe I can help.

Basically I'm patching the AddToCart button to add extra parameters to the rpc call since I have added extra inputs to the product page. I had to do this because I had to override the controller in the backend and control the variant creation process.

patch(WebsiteSale.prototype, {
async _onClickAdd(ev) {
const form = this.el

ev.preventDefault();
ev.stopPropagation();

const res = await rpc("/shop/cart/update_json", payload);
},
});

So you want to stop for a second while adding a product into cart, right? Share me the xml snippet or requirement here?

Everything works as far as what the add to cart button does, I call the backend and add the product to cart, I just need to show the cart notification when the button is clicked

Ok like mini cart open when we add the product into cart?

Yes exactly the notification you get at the top right of the screen

Ok for that, you need to changes inside the JS and XML like below(just sample):
<template id="product_review_form" inherit_id="website_sale.product" priority="20">
<form id="valid_review" t-attf-action="/submit_review/#{product.id}" method="POST" class="mt-2">
<input type="email" class="form-control email_submit" name="email_product" required=""
data-fill-with="email"/>
<button type="button" t-att-data="product.id" name="send_email" class="btn btn-primary mt-2">Send Email</button>
</form>
publicWidget.registry.AdvanceFormSwitcher = publicWidget.Widget.extend({
selector: '#valid_review',
events: {
'click button[name="send_email"]': '_sendEmail',
_sendEmail(ev){
your custom code here for open a popup..
}