How to dispute an in-app purchase?

I've made an in-app purchase and I feel they are not fulfilling their service.

This was supposed to be for a full year of syncing of my checkbook between my iOS devices but they not only failed in this task but I have also noticed other users have been able to see and alter my checkbook data. How can I get a full refund for this year's service? ($21.59)

All attempts to contact the app developer go unanswered.