I'm a web developer but I am a partner with a company that handles my client's SEO and they have a unique offer for e-commerce websites that to my knowledge I've never seen another marketing firm offer.
They guarantee you first page rankings for keywords that are related to your store or business, without any cost upfront. You only agree to pay once you receive the keyword rankings that are promised to you.
If you don't rank, you don't pay.
The firm is called Search Advisor SEO, and they do the keyword research to determine if they can get you ranked for high traffic search terms that are going to drive more business to your website.
Here is a promo link if you'd like to check them out: