The posted coords will bring you to a post office. You will not find the cache there, but it's a 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 6:00 a.m. to 5:30 p.m and Saturday 6:00 a.m. to 4:00 p.m. It is closed on Sundays.
When you go into the lobby, you will see the post office boxes on the opposite wall. I have chosen to work with only the far left bank, numbered 1.
Banks are a block of boxes that have a section of wall separating them.
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 digit of the P.O. boxes referenced to create the last 3 digits of the north and west coordinates, so that:
N 42 35.[(B1G1R5C4) + 3] [B1G2R1C3] [B1G3R4C1]
W 076 10.[(B1G4R3C4) + 6] [B1G4R3C2] [(B1G5R2C2) + 3]
Please be respectful and stealthy at both stages.
Please note that the residence next door is owned by the organization. Do not park in the driveway but you may walk down it to retrieve the cache.