Make it complete, easy to find, and focused - to the point. Confirmation emails should be sent when order is received and order is shipped. Be ready for returns. This is where things get hard. Its easy to create a web site and take orders.
Happy customers come back.
People who have purchased on the internet will be loyal customers if they get the service they expect. If they are satisfied, they will come back without searching for a better deal. If you are small, don't overspend on the site. The client said the site paid for itself within the first 2 months. Save your money for good backend processes and for marketing expenses.