When applying discounts, should we order them somehow? Or calculate the new price independently of other discounts.
Now if for example a 10% discount is first in the list of discounts followed by a 10$, the total sum will be smaller than if the 10$ would have been first. See BillImpl#getPrice()

The price of a booking is never set. This should probably be set when assigning RoomType to a booking? When fixed this will affect some of the tests (Check out amongst others), which needs to be altered by depositing the cost of the booking to the credit card which pays the BookingBill.

Missing managers;

• AdminKeyCardManager
• AdminDiscountManager

When registering key cards on a room and later retrieve them via RoomManager#getRoom(room)#getAllowedKeyCards they do not exist. See CheckInTest#testCheckIn() line 175, 176 (uncomment them)