There are two post office branches on the Cornell campus: one in Kennedy Hall (D5) and one in Ives Hall (D5). They close at 4:00 sharp. The downtown branch office, formerly the main office, closes at 5:00 pm, and is on Tioga street between Buffalo and Seneca streets. The main post office for the Ithaca area moved several years ago to its current location off Warren Road near the airport, much to the regret of downtown residents.
Several delivery services also have offices near the airport. Both Federal Express and UPS have walk-in services during the day at these locations. FedEx and other overnight delivery services will pick up on campus during the day. When having packages delivered, it is often a good idea to give your campus address rather than your home address in case a signature is required, though you should check if you are permitted to have personal mail sent to your department, as not all allow this.
On the commons is the UPS Store (formerly Mailboxes Etc.), which provides a range of shipping and packaging services. You can purchase all of your shipping supplies right in the store, and you can even rent a post office box from them. A similar business is open in Collegetown, most recently named Pak-Mail.