Well unless the substrate package is kept warm and humid, mites are most likely not going to lay eggs on it. Mites need a humid warm environment which is why snake cages are so great for them. Also mites are going to be more inclined to lay eggs somewhere dark and humid in the cage rather than in a cooler drier atmosphere. Also just order your substrate. I order ReptiChips and it takes about 5 days to get to me. No way snake mites are going to hitch a ride that long in dry, cold climates with no food. They will die before they get here.