Hi Jade, thanks for booking and I wish we could have helped more. Unfortunately, we're not a hotel; when a guest stays later without arranging a late checkout beforehand, it can really affect other guests' experience. In this case, the three girls arriving after you had asked and already paid for an early check in, and having been traveling since 4am they were pretty tired. Because your friend overstayed, I wasn't able to provide the early check in, the cost of which had to be refunded to them.
My cleaner also had to leave and come back later, which meant she incurred additional travel costs and missed taking another cleaning job that day where she could have made money. She works very hard, does a great job and has four young kids; I always try to treat her respectfully and make sure I don't waste her time. For these reasons, I had to charge you for refunding the guests early check in cost and the cost of my cleaners' extra travel and missing her other job that day. Your additional charge of $50 was to cover these costs; I have no flat 20% charge for overstaying. Since you only booked for one night, in this case it did work out to 20% of your booking cost.
I hope you can understand now how you incurred the extra charge. Staying in an Airbnb is different from staying in a hotel; you are staying in someone's home and being a good guest means respecting their requests for how you treat their space. I hope with time you will join the many, many enjoyable and respectful guests I have been happy to host with Airbnb: it's a great community!