What I did (until a couple yrs ago when all hell broke loose in my family and took most of my time) was to offer a free product to any of my customers that had purchased several items from me during the course of a year.  

And I have been on the receiving end of an offer like that a few times, from vendors offering their own thank yous to their customers.

But if you have tons of customers it has to be difficult to do anything like that.  And I agree that emailing or site mailing a personal thank you does not sit well with a lot of people.  They dont want to be bothered reading all that, lol.  They just want to make a purchase and be able to get some customer support from a vendor in a timely manner.

Some people dream of worthy accomplishments,
While others stay awake and do them.

I am a dreamer