Rest assured, the room is booked, the hotel just hasn't input your details into their system yet. Your card type is not accepted at the hotel. The card expiration date precedes the reservation date. You have reached your credit limit. There is a computer error.
Many rental agencies do offer car accessories such as child seats, ski racks, tape players, etc.
Yes you can, however, some modifications may cause rate changes. Car rates can change daily, even hourly, so any changes you make to your reservation after it is confirmed will affect the rate at which it is reserved. No, taxes and insurance are not included. Many rental agencies do offer shuttle service. You will need to ask about this at the pickup desk.