Have a Backup Payment Option Handy
We've seen this happen with very pre-order or release day we've been a part of. At least one person on our team or one family member cannot order because their credit card will not work.
When you buy a high-value item from a retailer you don't normally shop at, or even when you simply buy something of this price, it may be flagged as fraud. This matters more if you plan to buy the Galaxy S8 or S8 Plus off contract, but it can be an issue no matter how you plan to pay.
If you use a debit card, you should prepare to have a backup credit card handy. It's also worth keeping in mind you may need to make one last device payment plan.