you don’t necessarily need to steam your buns. if they are fresh, it’s warm out and your dog is hot, they should be pretty darned soft and nice. 15-20 seconds in a foil wrap to serve is perfect. warm them in the cooler weather so they open easier without breaking. you should have some time to figure that out.
an empty case of chips holds 6-8 dozen nicely and can be stacked. if you can easily buy buns, do it often. store them in the AC/dry air when not on your cart. set the package on your cart in the sun to “steam” them, but only do this with what you will sell that day, otherwise keep them as cool as you can.