I had a hell of a time getting paypal and ecommerce to work, and it seems all the documentation is spread out so I figured I would post what worked for me. Seem others had the same issues.

I was not able to get paypal to work with the supplied PayPalPayment.php which is located in the ecommerce/code directory (so it would be http://www.mysite.com/ecommerce/code/PayPalPayment.php).

However there is an updated version of the file and can be obtained here:
http://open.silverstripe.com/ticket/1760

Upload that over your existing http://www.mysite.com/ecommerce/code/PayPalPayment.php

The next step is to configure your http://www.mysite.com/ecommerce/_config.php to add the paypal information you need.

This is what I added

Payment::set_supported_methods(array(
'PayPalPayment' => 'Credit Card (Paypal)'
));

// Set the email of the admin who looks after receiving the order emails.
Order::set_email('ross@3point7designs.com');

//Real account for paypal
PayPalPayment::setPayPalRealAccount('info@3point7designs.com');

//Test account for paypal
PayPalPayment::setPayPalTestAccount('info@3point7designs.com');

//Who to send test payment from
PayPalPayment::setPayPalUseTestAccount('info@3point7designs.com');

Then you must edit the /ecommerce/code/CheckoutPage.php

and change this:

class CheckoutPage extends Page{
   
   static $db = array(
      "PurchaseComplete" => "HTMLText",
      "ChequeMessage" => "HTMLText"
   );

...

to this

class CheckoutPage extends Page{

      
   static $db = array(
      "PurchaseComplete" => "HTMLText",
      "PayPalInstructions" => "HTMLText", //add paypal instructions
      "ChequeMessage" => "HTMLText"
   );
      

Hope this helps those who had/have similar problems. After doing these changes and going to http://www.mysite.com/checkout/?flush=1 paypal showed up

Thank you so much for posting this. I've been struggling with the PayPal issue for a client. I've implemented everything you've outlined. PayPal shows up in my cart, but when I get to http://www.mysite.net/demo/checkout/ I have the following choices: Credit Card (Paypal) AND preferred payment method (select one) VISA, MASTERCARD, AMEX, PayPal.

If I select PayPal in either case as the payment option, when I click place order and make payment I simply get a blank screen the following url: http://www.mysite.net/demo/checkout/?executeForm=OrderForm

Do you have any thoughts/advice on this?

You need to create a template the template file:

/ecommerce/templates/includes/paypalPaymentPage.ss

It should be a regular template page, with $PayPalInstructions and $Content in the content area.

I made mine just say "Redirecting to PayPal", and then used JavaScript to submit the PayPal form.

Hey there, i am following this thread trying to set up paypal as well but now, after iv made those changes no pages are able to load and it is saying

"Fatal error: Access to undeclared static property: PayPalPayment_Handler::$URLSegment in /home/howardgr/public_html/lynnescolours.co.nz/ecommerce/_config.php on line 31"

If I remove "PayPalPayment_Handler::$URLSegment . '/$Action/$ID' => 'PayPalPayment_Handler'" from my ecommerce/_mysite.php file then the site loads but I when I process payment it errors and doesn't get as far as paypal.

Any help would be appreciated!!