This is the official PayPal Express extension for Spree, based on the extension by PaulCC it has been extended to support Spree's Billing Integrations which allows users to configure the PayPal Express gateway including API login / password and signatures fields via the Admin UI.
This extension allows the store to use PayPal Express from two locations:
Checkout Payment - When configured the PayPal Express checkout button will appear alongside the standard credit card payment options on the payment stage of the standard checkout. The selected shipping address and shipping method / costs are automatically sent to the PayPal review page (along with detailed order information).
THIS FEATURE IS NOT YET COMPLETE
- Cart Checkout - Presents the PayPal checkout button on the users Cart page and redirects the user to complete all shipping / addressing information on PaypPal's site. This also supports PayPal's Instant Update feature to retrieve shipping options live from Spree when the user selects / changes their shipping address on PayPal's site.
This extension follows the documented flow for a PayPal Express Checkout, where a user is forwarded to PayPal to allow them to login and review the order (possibly select / change shipping address and method), then the user is redirected back to Spree to confirm the order. The user MUST confirm the order on the Spree site before the payment is authorized / captured from PayPal (and the order is transitioned to the New state).
Setup your application
cp config/database.yml.example config/database.yml rake db:bootstrap
Go ahead and load sample data
Fire it up to see that it works
Shut it down
You'll need to have a Paypal developer account (developer.paypal.com) and both buyer and seller test accounts.
Your sandbox credentials are available from the API Credentials link.
Start your app
Name: Paypal Express
Now add your credentials in the screen that follows
review: unchecked 
Signature: signature from your paypal seller test account
Test Mode: checked
Password: API Password from your paypal seller test account
Login: API Username from your paypal seller test account
Add an item to cart
Address step: complete it using a valid US address. (Use Sean Schofield's from the railsdog site ;))
Delivery step: pick anything
Payment step: pick Paypal Express. If this does not show up as an option, repeat Step 3.
The Check out with PayPal button should appear.
Make sure you're logged into your paypal developer account in another browser window before clicking it, as you'll be redirected to your test account (same browser, new window or tab).
On Paypal's site (your previously configured Seller test account), log in as the Buyer.
If you set up a test buyer account as email@example.com previously, use this now.
You should now see the paypal order details screen with a Pay Now button.
Click Pay Now
You should now see the spree apps thank you for your order page
Check the payment
Edit your new order
Go to the Payments section from the right hand menu
Pending Payments should show Paypal Express with the options of Show and Capture
Click Show and look over the info available
The payment has status Pending with a successful authorization
Back to Payments
This time click Capture, then OK
Click Show to see what's changed.
You should now see two transactions, the previous Authorize transaction and a new Capture one with status Completed
To automatically capture funds, add this to you site extension's activate method:
if Spree::Config.instance Spree::Config.set(:auto_capture => true) end
 If you check the review checkbox in the admin section for Payment Methods/Paypal Express, the flow is slightly different. Instead of Pay Now on Paypal's order details page, it now says Continue. And the user is directed back to the spree app's Confirmation page showing a place order button. Use whichever suits your needs best. Personally, I leave review unchecked to cut down on the steps in the checkout flow.