You Can Buy AppleCare Later
AppleCare is Apple’s extended warranty service and it's something you'll want to look into. You don't need to buy it today though.
Apple is promoting AppleCare to iPhone 8 buyers but you don't have to buy AppleCare when you purchase your iPhone 8 or iPhone 8 Plus.
If you aren't familiar with the service and its benefits, it might be a good idea to hold off until you've done the proper research.
You should have a 30 day window to buy AppleCare which means you'll be able to pickup your iPhone 8 and then buy Apple Care later if you decide you need it.