Most hatcheries don't include heat packs. They include extra males, or have a high minimum for chicks. The box size varies for each hatchery. I ordered a couple of times from Ideal Poultry, and the first small order (7 bantams), they sent a small, single cell box (for up to 10 large chicks maybe?) and 5 extra males for warmth. For my next order of 21 chicks, they just sent a larger, double cell box. I'm sure they have boxes for all the different sizes of orders. For orders that are just over the box size, they might do the next box size up and include extra males for warmth (don't quote me on this though--I'm just guessing).
What hatchery are you ordering form?