Existing web site. Database has all the products that can be sold.
Products can be images (special pricing), documents (another pricing type) and common articles (pens, flowers, tee shirts... price depends on quantity)
I need the add to cart (session/cookies, whatever is best), cart summary and modification / checkout phase (register / already registered ? - db and tables exist), final phase (payment) with paypal (submit / payment / acknoledgement) and "order" creation in db.
I have a paypal account. the customers will use paypal to pay.