I signed up for square as I also have my own business so I can charge people online - I just set up a separate account for booking.com guests.
I've been learning as I go. Initially I never charged a prepayment, cleaning, or service charge but if you don't you are out of pocket and out a booking. I now charge 50% unrefundable prepayment of total cost. I then charge the remaining 50% 14 days before arrival. Sign up for square and you can charge a credit card - I only accept credit cards and advance payments so I know that someone will come. Otherwise they don't show up and again you've lost a booking and are out of pocket.
Square charges a service charge to process payments and refunds so hence I charge a 10% service charge to cover those costs.