I used to repair/refurbish old tube radio's/tube ham radio transceivers as part of one of my hobbies and I shipped them all over. The UPS clerk at the UPS store told me the very first time I asked to have "Fragile" stickers put on a box I was shipping not to do that. For the exact same reason you cited. Anything marked "fragile" is a target for abuse for UPS/FedEx/USPS, etc..
Did you ship with the tubes in the sockets, or did you pull the tubes before shipping?
Another thing I’ve been told about UPS...
It’s very hard to collect from them on shipping damage claims unless THEY do the packing.
And USPS:
It’s very hard to collect from them, period. I know someone who shipped an expensive musical instrument with insurance. The tracking died somewhere between two USPS facilities, and it was never delivered. The USPS refused the claim, even with clear evidence that it disappeared while in their custody.
You need to pack as if gorillas are going to be handling your packages.