Best advice, if its your song then you should have a link to a landing page on your site where they can a) download it for 0.99 from you directly, b) download it for 1.25 from itunes through tunecore or c) download it free with email content locking.
a) I say use your own ecommerce because you Will get money faster and see great income without waiting for check (assuming the song is still new and of value
b) most people might choose this option because its a service they are familiar with and might already have credits/gift cards
c) this is for the friends/cheapo people that want it but won't pay, so you help them and they help your pockets lol
Additionally you should urge others to try and add your songs to YouTube and use them in any video they want. If you go through tunecore and hqve the songwriters option, you will be able to get residuals from their content is servicing and streaming program! Trust me it helps