One thing I don't like about this mechanism is that my user would have to go through the dialogs required for each one of my in-app purchases and buy the free-trial version of each. Why can't I just keep up with the original purchase (download date) of my app, let them use all the features of the app for free, then after the 14 days, start requiring a purchase for the premium features in order to continue using them? You could display a custom "you are beginning a 14-day free trial of this apps premium features" to make your users aware this way they don't have to take any action on their part to start the trial.

One thing I don't like about this mechanism is that my user would have to go through the dialogs required for each one of my in-app purchases and buy the free-trial version of each. Why can't I just keep up with the original purchase (download date) of my app, let them use all the features of the app for free, then after the 14 days, start requiring a purchase for the premium features?