Hi,
I am a web application developer. You said that any platform will suit, if so I can develop a webbased application for this purpose. We can either run it in your webhostig if you have one or else we can install it locally in your system. If we use a web based software, the advantage is it that it can be used in any devices, anywhere at any time.
In this software, you can login enter users data, it will do the desired billing calculations and other features you require. A very user friendly UI will be developed.
This is my portfolio: [login to view URL]
I would like to have a quick chat with you to know what all features you require in the application.
Best Regards,
Abhialsh AR