Today the USPS is empowered to suspend the PES, if it believes such a private postal service would be in the interests of the general public.
Records of pick up and delivery must be maintained for Postal Service inspection if the time sensitive exception is being used.
[1] One does not need to establish a private mail delivery service for the occasional commercial transport of a letter outside the mails so long as the rate which would have been due to the USPS is affixed in stamps, the stamps are cancelled in ink, and the date of receipt by the carrier or the transport of the letter, are noted thereon.
There are limited exceptions for special messenger services which deliver less than twenty-five letters for an individual or company per occasion.
In such case no postage need be paid or affixed to the letters; pick up and delivery can be from private residences and commercial businesses.