Mail order back then involved checks or money orders, which minors would not have had easy access to, or cash on delivery, which would have involved an interaction with a postal deliveryman.

Moreover, the point of these laws isn't to prevent any particular illegal sale. It's to eliminate the market and reduce the volume. In-person, cash transactions are difficult for parents to track. But most kids aren't going to risk their parents finding credit card charges from mail-order alcohol and tobacco vendors. If a large fraction of kids were actually using that loophole, then enforcement of those laws would be a higher priority, like it is with in-person sales.

Product bans do, in fact, work. For example, sports betting has skyrocketed since the Supreme Court overturned a ban on online gambling: https://news.harvard.edu/gazette/story/2026/01/sports-bettin....