grounding a mailbox - isn't it already grounded?
In all likelihood the mail box is already a real good ground as pointed out in the comments. Your body is accumulating the charge as you are in the presence of the power line but wearing shoes that insulate you from the ground. When you touch the mail box the accumulated charge on your body is transferring to ground through the mail box. 

There are several things you could try to work around this situation:


You could look into purchasing a static discharge strap and / or heel strap for your shoes. These are available for use in factories and service depots where people handle electronics equipment that is sensitive to accumulated static charge. If you had this type of strap inside your shoe where it was touching your foot or slightly damp sock and then extending to the heel clip where it touches the ground this would discharge you as you walk along the ground toward the mail box. 
You could look at attaching a resistor to the mail box and then to a metal plate that was insulated from the mail box. A resistor of say 1Mohm could be used. Then each time you go to the mail box you would touch the metal plate first. This would help to discharge your body before you touch the mail box. The resistor would slow down how fast the charge on your body dissipates and eliminate the shock sensation. 


A final comment.

Some people are more sensitive to this type of situation than others. There are even folks that believe that living in the near vicinity of a high tension power line is unhealthful. You may want to investigate and learn more about this subject. It may help you to fully access the situation and determine if folks in your household are adversely affected by the high tension power lines.