Have a Backup Payment Plan
It's important to have a backup payment option handy in case the card you use to pre-order your phone gets flagged by your bank or credit card company.
If you don't normally make purchases at Apple or with the card you're planning to use, your credit card company might flag the purchase and that could hold up your pre-order. It has happened to us in the past.
If you aren't prepared, it could cause a delay and prevent you from getting the iPhone 11, iPhone 11 Pro, or iPhone 11 Pro Max model you want.