Three factors: 1. Location - most processors at this end of the market are country specific 2. Customer location - a pay by phone alternative is no good if you are selling overseas 3. Volumes - Most micro payment processors have hefty up front fees that need to be covered.
try paypal, they have a micro merchant option where transaction fee is only 0.05. it's perfect for mp3 songs
Ditto with Paypal's micro payments service. Simillarly, Amazon offer a payment service called FPS. For transactions less than $10, they charge 5.0% + $0.05 for credit card... so only 10c on your dollar transaction!
The OP explicitly excluded Paypal. I admit that this is an area I haven't researched for over a year now but every micro payment option I have come across so far has either been country specific or involving an up front fee. Personally I use Nochex but as the OP quoted a dollar price I reckoned it would be no good as a UK bank account is required.
OP excluded Paypal because s/he didn't know about other payment processing rates Paypal offers. It is fairly new program, but I like online payment services are innovative in this space. Credit card merchant accounts are truely a rip off for card not present.