If you have blues on finding the nearest mailboxes nearest to you then its time you just bookmark this service which allows you to find the nearest USPS mailboxes, USPS locations and UPS stores. You can use your Address, City State and Zip code to find it more accurately. This is another Google Maps implimetation which is really impressive.
The best advantgae as I see is if you are on the move on car and need to drop a mail you can just navigate to this site and drop it on the next corner. You are free and at your convenience of finding a suitable place to park your car and drop the mail. They have even a mobile site which makes it faster and much more accurate for mobile uses.
Related : Find taxi, limousine or shuttle using real time position of vehicles in your city