I think cheapest and easiest depends on your situation. I've used FreshBooks[2] with PayPal for some online services. I've also used Kagi[1] too. I'm looking forward to Amazon DevPay[3] when it becomes generally available.

[1] http://www.kagi.com/index.php

[2] http://www.freshbooks.com

[3] http://www.amazon.com/b?ie=UTF8&node=342429011