Save a product manually

<?php

/* You'll have data from your custom form POST to this code */

if (class_exists('wpCheckout')) {
    if ($wpCheckout = new wpCheckout()) {
        global $Product;
        $Product -> save($_POST);   //posted data will go into the save() method

        /* You can print_r the errors variable to see what is missing */
        echo '<pre>' . print_r($Product -> errors, true) . '</pre>';
    }
}

?>