Earlier quoted context omitted.
If you want customers to be able to pay with a credit card, it will be difficult to get individual charge fees under $0.30. If you expect repeat purchasing, best to aggregate charges either ahead of time (aka "pre-pay") or post.
How does iTunes do it? They're doing $0.99 payments all the time.
- charging a 30% processing fee
- taking a loss on small payments
- bundling charges whenever possible
- encouraging people to buy itunes gift cards
- being big enough to negotiate better rates than normal people can.