Need Simple Shopping Cart Into Template

I need a simple shopping cart that can do this :

1. Click on Web Package
2. Add amount of package to cart ( at same time, calculate 50% of amount )
3. go to check out screen to review cart and hit pay ( will use paypal. )

a sample of this can be seen at:
http://www.d4you.us/signup.php?design=1

This simple cart script has to go into my website.

Please only bid if you can do this now and know PHP left and right.

thanks.

GFD