Staff need APP training

Staff need to get training on the Macdonalds APP and how it works. i was there last night and ordered a cheaper meal on the app with a free upsize. Howeverafter handing the phone to the cashier with the app open showing the deal she pushed the back button and the deal was gone. then she told me I must pay full price. She called the Manager and he just shrugged and said too bad. According to him the APP is not applicable to the drive through window but on the APP it clearly states in store of Drive through