This is what I do... but I am still experimenting.
I use pay pal buttons and mail order only. I set up a option to buy a cd or mp3 copy of my album. Paypal allows you to redirect a user, once they pay, to a certain page. I made that page a receipt with a link to download the mp3 version of my cd. If they only want the cd they get redirected to just a receipt page with no download option. I blocked the receipt pages from being indexed by crawlers also.
Its very simple and pretty easy for the customer.
You can view my example at
http://crmusic.info
just go to the order page to view.