You are absolutely right that no matter what sellers write on a parcel Aust Post will go its own sweet way.  My experience lately has been anything that AP can cram into the letterbox will go in the letterbox despite what has been written on the package.  If it's letter sized the only time I've had any AP postie come to the front door is when a signature is required...and that the moment that is not guaranteed as due to Covid AP sign on delivery.