When shipping xenias you could try the following:
Attach the small rock with the xenias to a piece of styrofoam securely. Float the stryofoam in a plastic bag so the there is no easy way for the frag to get flipped over. Fill the bag a full as possible so the xenias will always be in water even if the package flips and tumbles while in transit. Put styrofoam peanuts around the bag to hold it in place. You may have a better survival rate. shipped this way.
I guess you could also use a smaller piece of styrofoam to keep them afloat, and it would move as needed to keep them in the water, and keep the rock from tumbling on them and smashing the polyps... I have never tried this method, but I have transported other soft corals this way before with excellent luck. (8-10 hr car rides)
Best wishes,
Kelly