There are two kinds of First Class shipping: First Class Mail and First Class Package Service.
Within First Class Mail, there is a package type called “Large/Flat Envelope”. 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.
On the other hand, within First Class Package shipping, there is a package type we refer to as a Thick Envelope (3/4″). 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 includes tracking.
Even though USPS does not include tracking on the very cost efficient First Class “Large/Flat Envelope”, they do return a reference number when a label is purchased. We agree it’s confusing – to all of us, including your buyers. So, we have hidden it across the board: SHIPMENT HISTORY and Shipping Confirmation Emails do not include this fairly useless number.