How much to charge to add shopping cart?

I have a site that I am creating that will require a shopping cart. Basically add to cart buttons for each of the 100 tshirts, view cart, and order with secure credit card transactions. I am looking for a reliable person/company that can accomplish this and has done shopping cart implementation before. Can this be done for about $300? or is that too low? My boss asked that I find a reliable person/company to do this.