Not true.....
The counter clerk can only ask you if the package is fragile, liquid, perishable, or potentially hazardous, nothing more, nothing less.
You do not have to state, nor do they need, or required to know, that there's a firearm in the package. No markings of any kind on the outside of the box to indicate a firearm.
They do not need, or are required to have, an FFL on file for anyone. In fact, BATF does not want copies of FFLs in individual's hands.
Their job is to ship your package, not interpret or put their spin on BATF's rules. Make a copy of the BATF's rules on shipping and hand it to the counter clerk if they balk on shipping. Keep going up the chain of command till you find someone that knows and understands their own rules....