It is very difficult, and the problem is that really, when you look at the way the regulation is written, it says that you can't sell to minors. Not that you can't ship to minors. So for my money, I'd bet if you had ID check on the shipment and one went through it would still be on the business to pony up the cash (because the FDA isn't going to sue or fine the USPS, they're just the shipper).
I actually asked this question straight up to the FDA on their hotline back when they were still answering (I think they're too busy now to answer most questions, which is not a good sign). And the answer was very interesting, I was asking if we needed to provide some form of auditable evidence that everything shipped was to someone of legal age. They stated only and simply that we could not sell to anyone under 18, and that they placed no requirements beyond that. Which to me means on the other side that any evidence you make of 'best intentions' or 'best efforts' to ensure you don't sell to anyone under 18 simply will not matter. There is no process they are willing to hold up and say, 'if you do these things, we consider that a reasonable effort to ensure that you don't sell to anyone under 18 and you're in the clear'. They will simply nab you if they can with fines ; if they offer no process then you cannot forward any legal defense, because there is no process that is the standard - the standard is that it cannot happen ever.
To me this meant they would be doing age compliance shops at various stores and on various sites to see if they can catch someone shipping to someone under 18 (we've had a few try already, shot them all down). Again, not that they require any paper trail (which could give you some legal shield), just that they can't catch you doing it. Which to me means that if they catch you doing it, they don't care why it got to the minor; only that it got there and in that case it's your (the business who made the sale) fault. That's my take, but honestly, age compliance isn't that tough to get through, but like I said, we're probably going to go through a third party as we've caught a few ID's sent to us that I simply did not buy at all. So we're just going to confirm them all prior to sale.
The nervous nelly in me wants to do id check/signature at the door as well, but I'm super cautious. It wouldn't take many violations to make this into a situation that could shut us down, particularly if our process is wrong and there's a lag between the violation and notification to us (meaning that another 3-4 violations or more could occur before we had a chance to close whatever gap we had) because the fines are very, very progressive. We are already set up to weather some, but it would not take long before they really took a toll.