<?php
define('MODX_API_MODE', true);
require_once($_SERVER['DOCUMENT_ROOT'].'/index.php');
$modx=new modX();
$modx->initialize('web');

$miniShop2 = $modx->getService('minishop2');
$miniShop2->initialize();
$cart = $miniShop2->cart->get();

$resource_id = $_POST['id'];

foreach ($cart as $key => &$element) {
    if ($element['id'] == $resource_id){
        //товар есть в корзине
        return true;
    }
}

return false;