You have to have the snake immediately? I'm sure if they're reputable they'll hold the animal until the check clears. Generally thats how a transaction works, you pay for the item, then you get the item shipped to you, not vice-versa.
If I were in the breeder's shoes, I too would wait to be paid before shipping out one of my hatchlings to insure that you're good for the money, not saying you aren't, but thats also a "what if". Whats a few days wait?