If your request is approved and your design is approved the postal service will produce the cancellation device at no cost to you.
What are some of the things the USPS is agreeable to commemorate? According to Publication 186: Pictorial postmarks may commemorate a wide variety of local events such as fairs, conventions, philatelic expositions, parades, Black Heritage month, art shows, historical building anniversaries, opening day ceremonies for local sports teams, and the anniversaries of schools, universities, states, or towns.
Another nice thing about getting your own temporary pictorial is that you can help defray the cost of your event by printing up cacheted covers, franking and postmarking them to sell through philatelic publications as well as any of the online sites that include stamp related collectibles, such as eBay.
The very first official USPS pictorial cancellation was for the commemoration of the Headsville, West Virginia post office -- the oldest in the U.S. -- during the Smithsonian Institution’s 125th anniversary in 1971. The entire post office had been moved to the Smithsonian’s National Museum of American History. The interior of the post office can be seen on the USPS’s mail order industry commemorative of 1972.