Charging DOUBLE for Double Data Promo

Every time I purchase a double your data bundle on my vodacom app, I get charged the full price instead of the half price promotion. If I don't go through my invoices every month and pick up the mistake then Vodacom will happily take my money every month without saying a word. So every month I have to phone in and get them to credit me. It's a waste of my time and I feel like I am being ******! It's bad enough that you charge so much for data and it miraculously disappears but how many times and to how many people does this have to happen to before you sort out your systems?