Per inquiry by Semiretired, any effective method of preventing tobacco (or e-cigarette) sales to minors is acceptable to me.
Virtually all tobacco sales to minors have occurred in brick and mortar stores (or via an adult who purchases at a brick and mortar store).
While its possible, there is no empirical evidence indicating that minors purchase tobacco products (or e-cigarettes) via the Internet, telephone or mail order.
And since all states are required to conduct compliance checks using youth who attempt to purchase tobacco products from retailers, and since the FDA is now similarly conducting compliance checks of tobacco retailers, the percentage of tobacco retailers that are caught selling tobacco to minors has declined from more than 50% two decades ago to just 4% last year.
Several State AGs and several health departments that oppose e-cigarettes have conducted compliance checks on e-cigarette retailers (including some online retailers) hoping that they could catch someone selling to a minor (so they can issue a press release claiming that e-cigarette companies are marketing to minors).
So online retailers should be aware that the next purchaser may be a teenager who is working for a health department (although I'm not aware of any state AGs conducting compliance checks in the past year).