Whoever you were talking to may have had the nous to deprioritize OnTrac even though you didn't specifically ask for that to be done. It seemed to me like the form of words was necessary because I'd complained about AMZL_US quite a few times before anyone actually mentioned that it was a thing which could be done - but maybe they're just hesitant to deprioritize that specific carrier because, as an in-house organization, it probably offers better margins than a third-party shipper would.
It was so bad that I contacted support and told them if I would rather not be able to order something than to order it and have OnTrac touch it.
I can only assume they marked my account as fussy-about-delivery because I haven't had any issues since then.