First off, your support team is terrible. I've been argued with about this for weeks. Everything from "we don't notify marketplaces about letters" to "that's a feature that you'll need to request".
No, it's a bug.
When ShipStation uploads shipment info to Amazon for First Class Letter, Large Envelope or Flats it tells Amazon it's a package. Instead, it needs to upload as "First Class Envelopes".
This negatively affects metrics on Amazon because the only tracking exemption is for First Class Envelopes.
So every day I have to go into Amazon and change what ShipStation sent for each order.