There's no delay of you buy in-app. Nothing to receive by email or anything. You buy your tickets and they are there in the app. I bought our last tickets with MaxPass this way and it was very easy!
I wouldn't wait until morning of, though. Do it the day before or a few days before. That way there is time in case a weird problem comes up (app won't load, your bank declines the charges suspecting fraud, etc).
Note: You will get an email confirmation in addition to seeing the ticket in the app, which you could use if the app didn't work at the turnstiles.