If you have Apple Pay or Google Pay set up on your phone, you can use that to check out.
However, please be sure your current shipping address is correct in your Apple Pay or Google Pay settings!
The point of those services is to skip having to fill in all your info. But if you haven't updated your address since you've moved, or gone to school, or been deployed, or whatever, then the app will put in your old address, and then the package might ship to the wrong place.
So if you've moved recently, or are trying to send a package to someone else, please double-check your address settings in the app before using it to check out, or else check out manually.