Magento 1.9 – Fix ‘The Requested Quantity is Not Available’ Back Order Products

Magento does not deal well with this scenario: deleting a payment method that has been used. Your options from here are:

1. installing the old extension again from a backup or similar
2. adjust the payment method in the database

For the second approach you can use something like

UPDATE`sales_flat_order_payment` SET `method`= 'NEW_CODE'  WHERE `method` = 'OLD_CODE';

As NEW_CODE you could test using the code from the newer version of the extension and hope that it is backwards compatible. You could find it in the same table for an order placed via the new extension. If that doesn't work out you could also change it to one of the default payment methods like checkmo or ccsave.

As usual if you make any changes directly to the database a backup is recommended beforehand.

You are approaching your problem from the wrong angle.

I guess you are seeing a 500 error because of an uncaught exception. The quote and cart models throw exceptions if the product cannot be added to the cart.

Use try ... catch to catch this exception and react accordingly.

Example:

try {
    $cart->addProduct($_product, array('qty' => 5);
    $cart->save()
} catch (Mage_Core_Exception $e) {
    Mage::getSingleton('checkout/session')->addException($e, $this->__('Cannot add the item to shopping cart.'));
}