These are NOT offered or sold by the post office. They're now only available from authorized postal service partners.
USPS used to have Label 400, a tracking number barcode sticker, available in lobbies. On January 22, 2023 discontinued it and replaced it with Label 888. These allow you to use postage stamps to mail packages.
According to the USPS Domestic Mail Manual in 500 Additional Mailing Services, Section 7.2 states:
* Label 888 can be put on packages with postage and fees prepaid by ordinary stamps.
* Mail your package and get a mailing receipt by giving your package with Label 888 to a clerk behind the counter at the post office.
** In 2019, the Aviation Security Program (aka the Anonymous Mail Program) allows packages 10 ounces or less AND 1/2" or less thick to be picked up by letter carriers or put in drop boxes, but packages that weigh more or are thicker must be given to a clerk at the post office. Kiosks do not scan these, even if it's under 10oz and 1/2" thick!
* Label 888 can be used on USPS Ground Advantage (which replaced Parcel Select), Priority Mail, and Media Mail.
These let you use ordinary stamps to mail packages with, but still receive USPS tracking.