I am not sure if other Amazon sellers with UPS account have noticed this, but UPS is cheaper when you use Amazon Buy Shipping.
ShipStation will not let us use Amazon Buy Shipping through ShipStation unless it is a Seller fulfilled Amazon Prime order. We do not use seller fulfilled Amazon Prime, but still use Amazon Buy Shipping for our UPS merchant fulfilled orders.
ShipStation wanted me to post this message on this forum, because they do not believe that there are enough sellers that will use this options, so ShipStation wants to see "up votes".
Who does not want to save money?