Probably a lot. 4 to 6 steelbooks (maybe more) can still be shipped in those larger cardboard mailers (the kind you tear the strip to open that fold in at both ends) and they don't offer any protection because the steelbooks just barely fit inside.
When it comes to ordering steelbooks from any Amazon worldwide, being shipped anywhere worldwide, you will always get some kind of mailer.
You're always going to be taking chances importing from Amazon. That's just the way it is.
Steelbooks are steelbooks. There will always be a risk.
But what else can we do?