It’s been reported that ordering online can take up to 4 weeks for a final delivery of the product. We have grown so accustomed to huge lines at the Apple Store during release dates that going there is not even an option anymore. It turns out that many stores, though stock levels projections are always unreliable, have the new iPhone available for same day in-store pickup.
Anyway, if you still prefer to buy it online, timing for such an option should improve in 2018 according to Apple. That is, if you’re willing to wait and trust the projections.