For past launches when availability was a serious concern, the app has always worked for me (I've had the app open on my phone along with the link open in Safari on an iPad and the page open on my PC simultaneously to use whichever one processed things quicker. The app always won.)I don't think location services being on or off matters when pre-ordering. I might try and use the App Store app this year, people have said that it's a lot faster to get through when pre-ordering than it is when using safari. I will be using Apple Pay on the app as well, so I hope that makes it even quicker, if it works that is.