Just making sure I send case packs correctly

I am a small manufacturer and I sell my product (6oz automotive accessory) through FBA. I used to send boxes of 20, but as sales pick up I sent boxes of 40 and still need to send more. I just had a box of 40 arrive with some damaged units.

It would be ideal to me to pack 20 units in a case, and then pack 4 of those cases into a box. It would save a lot on shipping and reduce the risk of damage, because 20 units fit very snugly in the case.

I know I need to have consistent quantities in the cases, and no SKU mixing, and no more than 150 units, and no exterior barcodes, but is there anything else I need to think of? I’ve seen people claim that cases need to be labelled but I don’t see a requirement for that.