That happens a lot. Seems the temperature change from their origin source, then through transit, then to you will kill some. Plus those ones that died could have also have been old breeders and close to the end anyways. I have noticed a lot less deaths during summer time vs winter time.
As for humidity, normal house humidity is fine. Just watch to see if you get any dying during a molt(stuck in the old chitin) or if you notice adult males with deformed wings. Those are signs of too low of humidity but for the most part, dubias can tolerate the lowest humidity of the roaches. I personally just have their water dish of water crystals in both my breeder and feeder bins and they are all fine. Misting your bin when you got cardboard egg crates in there is just asking for problems or more work than is needed imo. If you are really concerned, just put a bigger dish of water crystals in there and cover 3/4 of the top with the lid from the tote.