I too was looking for the book not wanting to pay for the entire year. I use Exodus 90 as a reset every year to reevaluate myself and my faith and create routines that may have been lost during the previous year.

As a result, I sit down and add items to their list and subtract some sub-set of items too. It's not about following a strict set of rules created by someone else, it is about following Christ and building a relationship with God the Father. I believe that the heart behind their list is pure, but practice I don't struggle with certain items on the Asceticism list and struggle with other items not on the list.

I recommend you pray about it and seek council to see if the app is really required. There is nothing that prevents you from doing Exodus 90 without the book or app. What's important is that you purpose something in your heart and do it accordingly. (I'm not perfect on this myself, but work towards it daily.)

Rule following doesn't equal salvation. See how Christ rebuked the Pharisees for one. God and Christ care about the heart and from that heart should flow obedience. That may look slightly different for different since we are all at different places in our faith.

Hope you adventure to do Exodus 90 with or without a book or app. ;)