There are several post boxes along Main Street. They're attached to the lamp posts, and are painted the same color so they kind of blend in. They're an older style so most people don't even realize they function as real mail boxes. If for some reason you can't find one, just ask a CM and they can help you. They're also located in the other parks and in the resorts. But, as an FYI, don't plan on mailing any bills or anything from the parks. Everything takes longer than the normal mail service, so plan for an extra few days.