Re: Shopping Cart
Most of the websites on the web send you to an external location because they are in charge of the payment. Which is good, you'd do best to stay well away from credit card details etc...
if you use pay pal then you set up your own stock list as a database (or just static information on a page) then when the user click add to cart it goes to paypal where the carts are based, the items are listed in the cart according to th unique name you give them and the price is sent as well (so paypal don't keep a stock list and works soley on passed information).
when the customer is done they confirm payment and you are sent mail confirming the transaction.
IMHO this is better than trying to create your own cart and checkout system where you would have to store customer name address and credit card details, you'll have to worry about credit fraud and proove that you are taking measures to keep the data secure, there are also many other requirements to the dataprotection acts that you would have to follow if you kep any kinds of details.