Magento – Magento and PayPal Payments Advanced User Authentication Issue

In my experience, 10002 is a pretty broad error, meaning that any number of things could be wrong from certificate invalidation to incorrect API credentials; I've seen this most often in conjuction with setting up Website Payments Pro. If that's the case and you're using WPP, you'll need to contact Paypal Developer Support to have them enable WPP for your sandbox account. Yes, that's a pain in the rear.

However, if you're not using WPP and you're exclusively trying to set up Express - try the following:

In Paypal, navigate to PROFILE > MY SELLING PREFERENCES > API ACCESS

Find the option that says “Accept payments from your online shops before setting up APIs” - set this to ALLOW.

For more information on Paypal error codes, check out the following guide:

https://developer.paypal.com/webapps/developer/docs/classic/api/errorcodes/

You need to tell PayPal about the discount order total. You can do this by modifying the PayPal cart object that is passed.

Observe the event paypal_prepare_line_items (Mage_Paypal_Model_Cart line 318)

In your observer, you load the paypal cart as such:

$payPalCart = $observer->getPaypalCart();

Get your discount amount from the order

$order = $payPalCart->getSalesEntity();
$discountAmount = $order->getDiscountAmount();

You can then set the discount amount by calling updateTotal()

$payPalCart->updateTotal(Mage_Paypal_Model_Cart::TOTAL_DISCOUNT, $discountAmount);

And that is it, PayPal will now know about your discount. This should work for standard, advanced, and pro I believe - both express checkout and direct payments.