Improve the look of the scripts. You might want to consult sources on stylesheets. However, keep in mind that your customers want to see the products and make their orders quickly.
Improve the display by changing either or both of the displaycart function and the money function. You might want to make the money function always return strings of a fixed size.
Change the code to support currencies other than dollars.
Add images to the final Web page presented to the customer: thumbnails of each product ordered.
Add fields to each of the tables and do validation using regular expressions. For example, check for credit card numbers of the format four groups of digits.
Design, program, and debug a set of scripts for the processes of shipping the orders and billing the customers. This would start with reading the orders and ordered items table.