The door release is likely to be a very inductive load (electromagnet) - you want to rate the relay contacts significatly higher than 0.27A and/or consider adding a free-wheel diode across the door-release coil to prevent inductive spikes.
Photos of the set-up might provide useful information (how is it powered with 18V for instance?)
If you want to power the Arduino from this same supply that's another thing to consider.