Had a look at this on someone else's phone.
They really are treating customers badly.
Firstly you need to sign up to the app. Then you have to "activate" the offer. Then you need to read the terms and conditions.
Included is a huge list of items. Over 80 I'm told.
BUT, they only allow a MAXIMUM of 9 items and it's the cheapest items that are free.
Due to the massive number of items, the chances are you will buy more than 9, and amongst your items might be 79c rolls or 59c farls - and it will be these cheap items that will be "free" - and only a max of 3.
Nowhere near as attractive as they make it out to be and simply not worth the effort.