But is that a good bang for the buck? How about advertising on Amazon? Or should I spend my time/money working on seo?
Yes, I don't know, yes.
Adwords/PPC can work, but it does take work - and testing (i.e. it won't work when you first try it). Find the PPC books by Brad Geddes and Andrew Goodman to get started.
SEO is broken down into two things, on page optimization and link building with the second one becoming more important the more competitive of a market that you're in. The first fix for that is to read the link building forum here (go back at least a year, maybe two, read every post). After that, attend a pubcon. THere's one coming up in a week or two, there's another in November I think in Las Vegas.
The other things you mention are really web versions of traditional marketing, so you're instincts should serve you well. Doing a mailing to all existing customers pointing them to your website is a real good start. You may make some sales right there.