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.
Apple aggregates to a great extent but I think could go much further. It only bundles up purchases over several days or perhaps a week. Since it's minimum fee is $0.30 (30% of $0.99) it just about breaks even on single song purchases.
But Apple would be fine with the whole App/iTunes Store being a break-even proposition since it has consistently stated that its interest is in the hardware sales.
