Large Envelopes / Flats are serviced by USPS First-Class Mail. The USPS does not provide tracking on this service. You can contact the USPS for more information.
Difference between First-Class Mail and First-Class Package
This service includes a package type called "Large Envelope/Flat".
This is essentially a flat envelope you would use to ship documents. It is the least expensive service you can get with USPS through ShippingEasy and it is not eligible for tracking services. Learn more about Large Envelope Flats
This service includes a package type we refer to as a "Thick Envelope (3/4")".
Often used for padded envelopes, it can be used to ship documents or thin objects that are less than 3/4" thick. It can be a little more expensive, but it does include tracking.
Even though USPS does not include tracking on the very cost-efficient First Class "Large/Flat Envelope", they do return an unusable reference number when a label is purchased. We agree it is confusing to all of us, including your buyers. So, we have hidden it across the board: SHIPMENT HISTORY and Shipping Confirmation Emails will not include this fairly unhelpful number.