From the latest postal bulletin 22486
DMM Revision: Ineligible Flat-size Mailpiece Clarification
Effective March 5, 2018, the Postal Service™ will revise Mailing Standards of the United States Postal Service, Domestic Mail Manual (DMM®), in various sections, to clarify the processing category of certain ineligible flat-sized pieces. The DMM currently states that mailers whose retail or commercial flat-sized pieces do not meet the flexibility or uniform thickness standards must pay the applicable parcel prices. Also, mailers whose flat-sized pieces do not meet polywrap covering standards must pay the applicable parcel prices. Mailers must prepare these ineligible flatsized pieces as parcels. The Postal Service will revise the following: Sections 101.2.5, 201.4.7, and 201.5.0 to clarify that the above ineligible flat-sized pieces are considered parcels.
Sections 233.4.0, 243.4.0, and 263.1.0 to remove redundant text. These revisions will provide clarity and consistency in the standards. Quick Service Guides (QSG) 201c, 207d, 207f, 230d, 240e, and 260a, to include the aforementioned revisions. QSG 270a to remove outdated flat-sized pieces text. Although we will not publish these revisions in the DMM until March 5, 2018, the standards are effective immediately.
The engineer study must not be treating the USPS nicely.
Of course what they call a parcel when charging the mailer isn't always the same as what they call it when paying a carrier to deliver it.