This is a vacation rental site. I integrated the design with backend and frontend using PHP-MySql and jQuery. People can contact de owner, see availability calendar on each property. Property owners can book any date that is available and the system shows them who is booked for on each property they own. I did all the ecommerce implementation with paypal: Handling Paypal API, installing their secure certificate.
We like people to understand our software, thus we make it simple. Less is less and useful is forever.