Shipping yeast during hot and cold months can be a toss up whether it will survive. Why not compound the recipe yourself and buy at your local Brew Store? Or at least buy the yeast locally to prevent the off-chance it gets cooked (or frozen) in transit. The small ice pack won't help much.
WY3944 is a really nice yeast for Belgian Wits. I've been using Wyeast for years. Performs the same as or perhaps even better than White Labs because of the smack pack. Freshness is most important. When using liquid yeast, even if it's fresh, you'll need to make a starter for anything over 2.5 gallons.