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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.
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.
Looking at your Shipment History, I see that some of your recent shipments were Large Envelope/Flat shipments. It is possible that the USPS is not using the APV system for that type of shipment. For future shipments, I would recommend choosing First Class Package then Thick Envelope (3/4") and also entering the weight and dimensions of the package.
You can also reach out to the Carrier (USPS) directly for more information about specific shipments: https://www.usps.com/help/contact-us.htm