Can you put a chain and padlock on the gate, as even if you get a postbox on the outside, any couriers delivering parcels may still feel the need to come into the garden. I have found some parcels in some very strange locations, one was on top of the car port roof for nearly a week before we noticed it, and I am sure if there was a way into the garden they would have come in.
Yes unfortunately, if your dog bites someone, even a burglar, in your own home, it is your fault.