This is the most important step. Assuming that there is a demand for your product and you are competitively priced then the only thing that is keeping you from making a ton of money on the web is your ability to be found. This is also the most time consuming or expensive process to do properly and effectively.
Hopefully you have picked good keywords, and optimized your sites for the search engines. But at this point it can take several months for your site to get indexed in the search engines. Then after several months you may find that your pages don't rank very well. So you can then make some changes to your site and then wait about a month to see if your changes work. Then start this all over again for other terms, etc., etc.. This is a very time consuming and frustrating processes and while it can be successful for some people unless you want to spend hours and hours constantly staying up on what the search engines want and then updating page it is just not worth it.
Your other option is to hire a company to do it for you.
The Large SEO (search engine optimization) companies that I have spoken to wanted around $3,500.00 per month to optimize a shopping cart. If you operate a large shopping cart site and have the money then it may be well worth the money? But from the companies that I have worked with I just can't see that it is worth it. It just takes too much time investment for the gains that you get.
You simply pay the search engines for traffic that they send to your site for specific keywords. This will generate instant traffic to your site which should in turn also generate instant sales. Off course a lot depends on the market that you are in and how much competition you have. You also must have the margins available so that you can afford to pay money to generate the sales.