Re:Amazon bans incentivized reviews tied to free or discount...
by Alupis on Tuesday October 04, 2016 @05:42PM
(#12639748)
That's a custom label (ie. not stock from USPS), and is likely generated by the shipper's shipping software.
Apparently they chose to leave that field off the label, but it's still sent to USPS by the software.
http://static-s3.sokoloff.com/random/amazon/IMG_1880.jpg
On other labels it's after the "DF3..." ID.