The posted coords will bring you to a post office. You will not find the cache there, but it's a nice walk or short drive away. In order to find the cache, you must first solve a quick puzzle, which you will do by entering the post office and doing a little post office box counting. The lobby is open Monday - Friday 7:15 a.m. to 5:15 p.m and Saturday 7:15 a.m. to 3:15 p.m. It is closed on Sundays and holidays.
When you go into the lobby, you will see the post office boxes on the left. There are 3 banks one the left side, one straight ahead and one bank on the right side. I have numbered the banks from 1 to 3 with 1 to the left with the second one directly in front of you when you enter and going around to the right is number three.
Banks are a block of boxes that have a section of wall separating them or in this case on a different wall.
Grids are blocks of boxes that are separated by a thicker piece of metal, but they are adjoining other grids within the same bank.
Lastly you will need to take note of the row (horizontal) and column (veritical).
Take the last 3 digits of the P.O. box to be the last 3 digits of the north and west coordinates, so that:
N 42 29.B1G11R9C4 W 076 17.(B2G2R6C4+270)
Please be respectful and stealthy at both stages.