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 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.
On the other hand, within First Class Package shipping, there is 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’s 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.
What is a First Class Large Envelope/Flat?
What is First Class Package Service?
What is USPS Delivery Confirmation?
How to: Send a Shipping Confirmation Email to my customers
How to: Create a custom Email Template and save it to your store