Yeah, legally they are supposed to ask even if you go to Staples or Office Depot or places like that. The 2nd hand shippers tend to ask fewer questions. If I send USPS I vacuum seal all the bottles in food saver bags just because I'm scared to death something is gonna get on a plane and start leaking. Also vacuum sealing does a remarkable job of keeping the bottles from knocking together if they are in the same bag. It's still illegal and I'm not suggesting anyone do it.
The few times I did it, I said it was soda, only so they know what they are in for if they drop it or put it on a plane or whatever. Hopefully congress will pass the bill I heard about and all this will become a thing of the past but it has yet to be tabled for a vote as far as I know.