I was just on this website at http://www.offpricenetwork.com that has an Alexa ranking of 492,000 but is using custom Adsense links. I thought only websites with a huge amount of traffic could use custom links. I don't think they can be getting that many clicks with an Alexa ranking like that.
yes, they do look like a premium publisher. just because a site has low traffic doesn;t mean the publisher has only one site. there could be other sites where the publisher is running adsense and overall must be having enough traffic to quality as a premium publisher