// For debugging, this function is called by hook_menu()
function mymodule_test($product_id)
{
    global $user;
    $user = user_load($user->uid);

    $order = commerce_cart_order_load($user->uid);
    $order_wrapper = entity_metadata_wrapper('commerce_order', $order);

    $product = commerce_product_load($product_id);

    $line_item = commerce_product_line_item_new(
        $product,
        1,
        0,
        array(
        ),
        'cover'
    );

    $line_item_wrapper = entity_metadata_wrapper("commerce_line_item", $line_item);

    $line_item_wrapper->commerce_unit_price->data = commerce_price_component_add(
        $line_item_wrapper->commerce_unit_price->value(),
        'base_price',
        array(
                'amount' => 1234,
                'currency_code' => 'EUR',
                'data' => array(),
        ),
        TRUE
    );

    $insert_line_item = commerce_cart_product_add($user->uid, $line_item_wrapper->value(), FALSE);

    return 'done';
}