The Postal Service defines a "parcel" as anything that isn't a postcard, letter or flat (a large envelope, newsletter or magazine). A box of cookies, for example, would be a parcel.

Deutsche Post DHL to expand parcel, letter tracking By Reuters March 3, 20202:02 AM PSTUpdated March 3, 2020 ...

A new tamper-resistent postal box has been installed in front of the Ansonia Post Office on Main Street. The new box contains a slot for letters but no chute.