Hi,
The postboxes come pre-drilled. for easy fittings. line up where you want the post box to go ensure it is level. Mark the positions of the pre-drilled holes in the box.
Drill the holes, the black plastic spacers go between the wall and the back panel of the post box to provide a gap for rainwater to drain off the wall and also to level out any unevenness from the brickwork or rendering. In your case, if there is high wind, it would also be beneficial to use large washers [not supplied] behind the screw heads to provide a larger load area to prevent any damage to the box.

Hope this helps