It's a legal loophole where the seller requests the check but the person delivering it has no binding liability to do so and they simply will not because it takes extra time. The economics practically guarantee the check won't be performed and the interface mechanics of carrier-seller means there's no practical way to prosecute either party when the carrier doesn't perform a requested ID check.
https://www.periodpaper.com/cdn/shop/products/EM2_315_1200x1...
The last time I ordered some tobacco a major, non-shady, licensed vendor literally had USPS pick it up and drop it straight in my mailbox.
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....