I’m facing a hard problem in adding a product to the cart via api for non signed up user in WordPress. I started to think it is not possible to achieve it. It works properly for users which are signed up but what about guests?

Here is the scenario for reference:

1. Data gets sent from somewhere and it must include a total in body.
2. I get the data and add number of products === total.
3. Why? the product price is 1 so I got the total price again why all of this ? Let me say it is the easiest solution.
4. Finish the payment process.

Below you can find the code:

<?php
/*
Plugin Name: Receive Checkout Data
Description: Receives checkout data and returns checkout page URL.
Version: 1.0
Author: mo_sleem
*/

// Register REST API route
add_action('rest_api_init', function () {
    register_rest_route('custom/v1', '/receive_checkout_data/', array(
        'methods' => 'POST',
        'callback' => 'process_received_checkout_data',
        'permission_callback' => '__return_true', // Allow access without authentication
    ));
});

function process_received_checkout_data(WP_REST_Request $request) {
    $data = $request->get_params();
    $checkout_url = wc_get_checkout_url();
    $product_id = 58;
    $quantity = isset($data['total']) ? intval($data['total']) : 1;``

    if (isset($data['customer_id']) && $data['customer_id'] > 0) {
        $user_id = intval($data['customer_id']);
        $user = get_user_by('id', $user_id);
        if ($user) {
            WC()->session = new WC_Session_Handler();
            WC()->session->init();

            // Start a new customer session
            wc_set_customer_auth_cookie($user_id);
            WC()->customer = new WC_Customer($user_id);

            WC()->cart = new WC_Cart();
            WC()->cart->set_session();
            WC()->cart->get_cart_from_session();

            $cart_before_add = WC()->cart->get_cart();
            WC()->cart->empty_cart();
            $cart_after_empty = WC()->cart->get_cart();
            
            if ($quantity > 0) {
                WC()->cart->add_to_cart($product_id, $quantity);
            }

            $cart_after_add = WC()->cart->get_cart();

            // Save session
            WC()->session->save_data();

            return new WP_REST_Response(array(
                'url' => $checkout_url,
                'data' => $data,
                'cart_before_add' => $cart_before_add,
                'cart_after_empty' => $cart_after_empty,
                'cart_after_add' => $cart_after_add,
            ), 200);
        }
    } else {
        wp_set_current_user(0);

        WC()->session = new WC_Session_Handler();
        WC()->session->init();

        WC()->cart = new WC_Cart();
        WC()->customer = new WC_Customer();

        $cart_before_add = WC()->cart->get_cart();
        WC()->cart->empty_cart();
        $cart_after_empty = WC()->cart->get_cart();
        
        if ($quantity > 0) {
            WC()->cart->add_to_cart($product_id, $quantity);
        }

        $cart_after_add = WC()->cart->get_cart();

        // Save session
        WC()->session->save_data();

        return new WP_REST_Response(array(
            'url' => $checkout_url,
            'data' => $data,
            'cart_before_add' => $cart_before_add,
            'cart_after_empty' => $cart_after_empty,
            'cart_after_add' => $cart_after_add,
        ), 200);
    }

    return new WP_REST_Response(array('message' => 'User not found'), 400);
`}`
`
The code in response (non-signed up mode) returns: data of empty cart and add to it, by the way it doesn't modify a thing for the current one.