Well, to be honest I would start with a usual Marketing approach
In this particular case of a pure online shop, this might only be google, but google doesn't have to be your only stop. If your products are sunglasses consider fashion magazines (online or offline), a facebook site, ad on mobile devices or similar. Think broader and be part of your target group, then try to think where you would go, where you would search to find your product.
I know this may sound very basic, but these days it is the basics that are forgotten while we search for sophisticated solutions. Hope that helps.
Adwords is good but it will be very expense for a site like shopzilla... I am pretty sure they have some deal with Google. But first make sure your site is super fast, SEO friendly (titles, meta data, urls etc.), target some very niche products (hard to find ones) so that users will get hooked to your site and may bookmark it.
Shopzilla probably does very deep marketing on adwords. Have quite high man power to maintain those dynamic campaigns.