What everyone else said.
Also, make sure you are 100% ready for your baby when he/she arrives.
1. Proper sized (and escape proof) tank with proper temps (ground temps checked with a heat gun) sufficient heat sources ALL REGULATED by a thermostat.
2. 2 (minimum) appropriate sized and identical hides.
3. Water bowl
4. Proper food items available or in your freezer. Ask if your baby eats live or F/T. Keep in mind if you want to feed F/T and he/she currently eats live, that can be a difficult transition, especially for a stressed BP in a new environment. If he/she has only taken live, see if the breeder can offer a few F/T meals before shipping to ensure that you are offering continuity in the food department. If he/she eats mice, offer mice. If he/she eats rats, offer rats, etc.
5. Proper humidity in tank.
6. Appropriate substrate in tank and more available because you will need to spot clean often and replace every 1-2 months (unless using printless newspaper or paper towels). If using paper substrate it is easier to maintain a clean tank, although there are drawbacks.
For heat and humidity - dial in for at least 3-4 days, if not longer, before the animal is shipped. Make sure you have everything set up with minimal adjustments before he/she arrives.
Keep asking questions of both the forum and the breeder! Ask anything you wonder about or need to know to keep a happy healthy snake for a long time.