I used to work for a ecommerce company and heard some good things about zoovy. Get some referrences from them and find out what customers are saying. Most of these types of companies offer a package, but to get anything custom you are looking at spending some real $$, so ask what they charge because it never fails that after doing business for several months you are going to want to tweak your product page or optimize your shopping cart or send feeds to different shopping comparison sites.
I would recommend sticking with Authorize.net for your credit card processing. Authorize.net offers instant fraud checking and sometimes even buyer protection which is key for your shoppers.
I would also look at getting an SSL Certificate and getting an account with HackerSafe and BBBOnline. These certificates help your customers feel at ease while doing business with you.
While I`m on a role, make sure that the number of steps to purchase your product is maximum 5 from clicking a product to confirming the order. This can usually be done by modifying your checkout to one page. This helps keep customers from falling off the cart.
Also, I would highly recommend putting some analytics on your site. This helps to tell you where your customers are going and what products they are looking for. Once you know that you can put those products that are most popular in an easy to locate place on your website (usually the front page). I highly recommend Omniture SiteCatalyst. It`s pricey but worth it IMO. If you can`t spend that kind of money right now think about using Google Analytics, it`s free.
chris.smith at procertify.com