Never had problems with the usual cardboard box. But for overseas, it's not strong enough. And they don't use bubble wrap nor foam to protect the steelbook.
It's better than Amazon's packaging, though Amazon customer service is better should something go wrong.
Trick is to order several titles together and pay a couple of extra bucks for gift wrapping - €2.25 to be precise for anyone wondering, that's for the total order, not for each item like Amazon - it's not much extra protection, but that way they're snug and arrive fine. I've only received once a damaged steel and it was more than likely packed that way as the box didn't show any signs of being dropped or hit.
This is always a risk but I've never found their boxes to be 'flimsy,' per se. But then again I've been lucky so far with FNAC (knock on wood). I've had a couple arrive dented - they might have been damaged prior to shipping since I've never received a damaged box from FNAC - but almost all of the steelbooks I've purchased from them have arrived in mint condition.
I only ordered from once. received damaged. Shipped very poor. No packing. It was to the US. And I had to do a Bank Disputer to get a refund. I won't buy direct from again.
It's not exactly scientific, but you can currently add 999 copies to your basket, which would lead you to believe they have over 1,000 copies available to order.