To use PayPal, you simply have to add it as a payment option under account settings. Make it the default if you'd rather use it than your card on file, and it will work even if you make your purchase through an Apple TV or an Apple Watch. It will even bring PayPal's One Touch mobile feature to those devices, which means you only need to authenticate transactions made through the TV or the Watch with your fingerprint. Cool, if that makes things more convenient for you, but dangerous if you have the tendency to buy impulsively.

Update: Changed parts of the story now that the feature is live in 12 territories.