If you want to go down the pay-to-download street, there is an open source webshop solution called osCommerce
that is very flexible and extendable and has an active user community. I use it for a few shops and I'm very pleased with it.
Among other things, it supports downloads, so that if you are, say, a software house, you can let people buy your software and allow them to download it directly after payment has been made. It supports PayPal and credit cards, among others, and is easy to set up. It's not kPlaylist in any way and it never was intended to be - it's a webshop system. However, you can let customers purchase kplaylist access by importing their user accounts from osCommerce into kPlaylist and allow them to stream/download music from your server.
You would probably have to write a script to do the importing, and probably also some sort of counter or timeout that would count the number of times bought songs have been downloaded/streamed or to time out the account after a number of days.
Personally, I think just offering the songs for download through the webshop would be the easiest, but I'm a slack bastard.
All I'm saying is that there are solutions out there, but you need to have a look at them and decide for yourself.
Best of luck,