A post office is a dilapidated, dirty building that is owned by the government and staffed with angry union workers. You can stand in line there for a good part of an hour to purchase an overpriced shipping service. You can purchase envelopes for an extra fee. Once your package is sent you have no record of its existence until the recipient tells you that it arrived. There is nobody to call regardless of the need. An option exists where for extra money you can purchase a number, and you can track your package by that number once per day, whenever the USPS worker can be bothered to scan the barcode.
Alternatives include FedEx and UPS. These services send you free envelopes and labels; you can set up your account on their Web sites and print shipping labels on your computer. When you are ready the driver will stop by your place and collect the package for free. The driver will typically not curse you for using their service. Once the package enters the UPS system (or earlier!) the package will be scanned and tracked whenever anyone touches it, and you can track it in real time. In case of change of plans you can call these companies and talk to a live person who will assist you.