I'm really shocked to see that Amazon Buy Shipping doesn't have the Ground Advantage option available yet. The first-class service has technically ended for packages, but the USPS says they'll accept it until September 30th. That doesn't leave much time to fill the gap. I would have thought this was such an easy fix.
Most packages we ship are First Class Mail, and we always use Amazon Buy Shipping to protect ourselves. This service is available on Amazon itself. I would urge the Dev team to persuade Amazon to make it available to them. This really complicates things as we would technically have to ship some orders inside Amazon and some in ShipStation. What a mess heading into Q4.