If you want to mail post cards or anything else, a post office is available at the end of Wadi Musa and right by the gate to Petra. It is attached to the building the visitor's center is in. The attendant put all my stamps on - by licking them and slamming his fist down onto the stamps. Thanks, man.
Fondest memory: Trying not to get a headache while he slammed his fist onto the desk and stamps on my postcards 30 times.
Favorite thing: Not far from the Petra Gate (2 km) is this ATM, which is the closest you will find. There are none nearby the Petra Gate. Be aware though that it will ask you what language you want - Arabic or English. Both choices are written in Arabic though!! Press the bottom right button and it will continue in English.