Glad your store is steering you right so far. Definitely do not bring him home until a few days after he's eaten for them. It can be hard to get hatchlings feeding, and he's almost definitely a hatchling (which should make him 15" or so long). Aside from leaving him there until he's eaten and digested, the best advice I can give is get the aquarium set up with the proper temps and humidity before you bring him home, to avoid unnecessary stress for both of you.

And congratulations on joining the ranks of ball python fans. They're wonderful snakes.