I signed up from www.whatever-domain.com, but I have dozens of other sites. Can I send traffic from those sites too or do I need separate accounts for each site?

You need to provide a website URL when signing up to our program. However, if you have other domains, you are more than welcome to send traffic from those too without permission (you are of course bound to only use sites with legal content!). You can use the custom campaign codes to track sales from multi-site traffic for productivity.