I'm not sure if they are called, "heated hand pads", but there are heat packs you can activate by breaking an inner pack in the bag. I believed they were made to be used for human injuries, like a portable heating pad. When my babies arrived from MyPetChicken, they had a heat pack in there for them. It was still warm when they arrived 24 hours later!
I took some (about 50) three day old chicks to a swap - and I had them in a vented box - I was afraid they would get cold - their own body heat had it so warm that I stuck my hand in there and was sweating - the outside temp was only about 55 degrees.
Depending on how many chicks you have they will keep each other warm, to a point - but if you sell a lot and have to have only a few left, then yeah, I'd go with the hand heating packets (but save them until you find out if you need them, or the babies might overheat.)