These are some links that helped me create my power point about the Post Office. Enjoy!
Topic: The Post Office
Grade: Pre-K through 1st
The United States Postal Service: www.usps.com 
This website is the official website of the      United States Postal Service. It provides useful information for postal      service customers, such as a shipping rate calculator, a post office      locater, and an online store. In addition, it provides a timeline      detailing the history of the post office as well as a discussion about how      the service began.
Canada Post: http://www.canadapost.ca/segment-e.asp
This website describes Canada Post, the Canadian      postal system. Students can log on to this website to compare      Canadian mailing services with that of the United States.
Stamps 4 Kids: http://www.bumperland.com/stamps.html
This website describes philately, the study of      and collection of stamps in terms that are easy for kids to understand. It      provides tips for students interesting in starting a stamp collection and      also gives useful resources to help build a stamp collection.
NationalPostalMuseum: http://www.postalmuseum.si.edu/exhibits/2_exhibits.html
The official website of the NationalPostalMuseum, a museum that is part of the      Smithosonian in WashingtonD.C. The website gives pictures of      the museum, lists permanent and temporary exhibits and provides resources      for those interested in stamp collecting.
NationalPostalMuseum: http://www.postalmuseum.si.edu/activity/8_activity.html
A special section of the NationalPostalMuseum website just for kids.      Includes trivia games, a create-your-own stationary activity, and online      coloring book. A great site to have open during      computer free time.
E-bay BiddingWebsite: www.ebay.com
Students can log on to this website and conduct      a search for stamps. They will be able to compare how much      collector stamps can cost as well as compare how much stamps cost in      different countries. Conducting this search will also help students      understand the great plethora of stamps available.