Echoing Simes, here, AJ. If it's a rolled poster, be sure they use one of the gorilla-proof tubes like Yazoo Mills or an equivalent thickness and sturdiness.
And if a folded poster, ask them not to place between sheets of corrugated cardboard (which bends) but solid pieces.
Most of what I have ever received has been thru USPS and, knock on wood, has always arrived safe n sound, even when the packaging was less than stellar.