For the time being, this library depends on the Zend library being included and autoloaded. I've only tested it with 1.11, but it probably works with others too.
$merchant_id = '123456789123';
$merchant_key = 'jklFJKLljkJFjkllKJl';
$order = new GoogleCheckout\Order();
$order->setMerchantId($merchant_id);
$order->setMerchantKey($merchant_key);
$item = new GoogleCheckout\Item();
$item->setName("testitem");
$item->setDescription("testdescription");
$item->setPrice("150.00");
$item->setQuantity(20);
$order->addItem($item);
$redirect_url = $order->getRedirectUrl();
If you want to make any changes / add any new features, by all means send a pull request! I only ask that you provide unit tests (I can help with these if you've never used them) and make sure any documentation associated with your contribution is up to date.