It worked for me! I was also told at the post office that such tracking numbers were for "internal use only", but apparently not.

As an aside, I routinely send USPS 1st class packages (padded envelopes with a chip or two in them) for $1.93 using PayPal to print the mailing label (and postage). Cheaper than taking it to the post office, and of course you do get free tracking and free $50 insurance.

I asked at the PO if I needed to bring such postage-paid packages to the counter if they weighed over 13 ounces (weight of the smallest practical bomb, I guess). I was told no, as the USPS already had identified me when I printed the label and charged my PayPal or credit card for it.