How to refund for Mantano Bookari Premium?

I've used to use Bookari Reader before on Android device. Remembering that, I've purchased its (Premium version) for my iPhone 4S as a decent PDF reader.

I've opened epub document which I used to read on that Android device. Unfortunately, when I try to navigate to some page - the reader just hangs... Then it restarts... Then I try to navigate - no luck, hangs and restarts.

Since I see Bookari is useless for me at my iPhone - I've decided to refund 4.99.

I've contacted Mantano (submitted online form) regarding this issue with asking to refund. Ignored.

I've submitted issue via "Report problem" on Apple site and it told me the given item was not refundable.

How is that?? Why I should pay for the app which I don't like and don't use? I've uninstalled it now and I think it's fair to get a refund.

Could someone please suggest me what to do?