I'm hearing some bad stories about the USPS. !
Build a wooden crate & line it with cardboard & screw the sign down thru it’s mounting holes.
just had a sign shipped from 25 miles away & it came damaged shipped by USPS -
some of the blame would be the sellers poor packaging + the shippers throwing around the freight
as Chris posted, you would be assured it would arrive safe
I've had the most luck with fed ex