I still put the sample in a box and send it Priority mail at my own cost. Always arrives in the typical 2/3 day delivery window.
The one of 2 times I did not put it in a box, lady at post office put it in for "Ground Delivery" or something like that when she asked and I told her it was motor oil. Even though motor oil is not hazardous many postal employees do not know their own policy and I think she marked it hazardous.
It took forever to get there.
With that said, one other time I shipped it in the bottle without a box and it still got there in the 2/3 day window period but the postal employee didnt ask if it was a liquid.
So bottom line, a box always got delivered fast for me, shipping in the bottle is hit or miss. Maybe the sorting machines and bottle are another possible issue like you are having.