Actually, I was building ecommerce systems several years ago and we took it as dogma to hit the db a few times each page. Having the product descriptions in the db is the most important thing, for flexibility of display and back-end inventory and product maintenance.
But if you have less than 10 products, then no, you don't need any of this.
Learn cookies and make a shopping cart that puts everything into a cookie, sending it along from page to page as the user browses the store. The cookie is *basically* just a raw string of your own formatting, all you need is pairs of product_id and how many. When they are done, have the PHP script email you their order, or dump it into a plain text file, xml file or comma-delimited for importing into Excel.