This is ridiculous for a low-mileage car like yours. Sorry but they talked you into throwing away $50.
As long as you do regular oil changes and don't make a lot of very short trips your engine should never build up a significant amount of sludge. And even if it does, after accumulating much higher mileage, a flush would actually be dangerous because the loosened sludge could find its way into the oil pickup and clog it causing oil starvation. Also dissolving deposits around older engine seals can cause them to leak.
If this place values your business maybe they will give you your money back if you convince them you didn't need the service. You may want to consider changing your own oil in the future, it is fairly easy to do.