I didn't have this in the version posted above, because the package involved, showed delivered several days before. But often, if a person is complaining and the package shows delivered the same, or previous day, a line like this is also helpful, toward the beginning of the letter, because it is often, the truth.
"Since tracking shows your package was delivered today, please give it a day or so, because the USPS has recently been notorious, for scanning packages as delivered, before the delivery is actually made. This is done in part, to keep their on-time percentage numbers, in line. If this is the case, your package is safe, just not yet delivered to you as of yet. Generally, when this is the case, you will receive your item later in the same day, or the next day, at the latest."