What the rule actually grants
The Postal Service describes Street Addressing as an election customers in competitive locations may make: the option to use the Post Office street address for their mailing address along with the customer's box number preceded by a hash mark. The consequence for parcels is stated directly, that customers choosing the designation "also have the option of receiving packages from private carriers at the customer's Post Office Box address, if the packages comply with applicable Postal Service standards governing mailability", per DMM 508.4.5.4 at pe.usps.com, checked 2026-07-28. The Postal Service's own PO Box page names UPS, FedEx, DHL and Amazon as examples.
How the address is written
Publication 28 section 284 describes the shape: the Post Office street address for the mailing address, along with the PO Box number preceded by a hash mark or a UNIT designation. The example carried in the recipient services rules looks like a normal street line, reading John Smith, then 123 Main Street # 4567, then the town, state and ZIP. Remember the spacing rule that applies to any secondary number written with that symbol, and prefer UNIT where the holder tells you their office uses it.
The limits a sender cannot check
Availability is not universal. The Postal Service states the service is not available at all Post Office locations, ties it to competitive PO Box service rather than every box, and requires a customer agreement, per usps.com and DMM 508, both checked 2026-07-28. There is no public lookup that tells a sender whether a given box is enrolled, so the answer has to come from the recipient. Once it is enrolled, our checkout treats the destination as the ordinary street address it now is, and screens it against carrier records like any other.