I would use Amazon EC2. Just run the instance, copy the script into the instance, add it to the startup procedure and add the completion to the end of the script. Link this to your own image and turn off the instance. Now you can run your own image every month and it will execute the script and shutdown. You pay only 10 ¢ / hour with an instance of m1.small.
You just need someone to manually start the instance monthly or the client set a small monthly cronjob to start the instance.
Note: shutdown -h is required, otherwise the instance will reboot and not stop
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