PayPal Express button is displayed on 'Select payment method' page (if enabled). We could not redirect a customer to PayPal site from shopping cart page if the customer haven't selected a shipping method, billing/shipping addresses.
The "cool" think about Pay Pal Express is exactly that: the user don't have to select all these things. Instead, it's redirected to Pay Pal and can use the data stored in his/her pay pal account or enter them again if he/she wishes.
I've seen that Pay Pal integration could be extended a lot. Mainly:
- button on first page of checkout
- passing customer language
- passing cart information and description to display on pay pal page
The can make a big difference reducing cart abandonment.
