Now, about your other questions. I wanted to limit the app only to a phone, because a phone is almost always online, either via Cellular or Wifi. Also, the UX is designed for a mobile screen not for the larger iPad screen. For these reasons mid-way I had to cut iPad support. (I need to remove iPod and vision, thanks for the heads up.)
About free cost and a one-time payment for senders: I didn't like the subscription model because I just wanted people to pay once for the app and never pay again. However, I didn't want to have two apps, one for senders that costs a dollar, and another for recipients that's free. Having two apps named almost the same would confuse the recipients. Therefore, it just had to be one app and I decided to use the in-app purchase method.