Magellan
Sometimes I've had problems navigating airline websites but I've just done a dummy booking for CAX-SEN-CAX on 14th and 17th February and encountered no difficulties. The only thing I find that's not completely intuitive is that you have to click on the flight details panel for the date chosen in order to bring up the available fares, but apart from that it seems straightforward enough. The return fare for the dates I selected was £113.70 per person which I don't think unreasonable. Maybe try again and see if you have better luck with the website this time.