Magento – Add products to order programmatically and have payment module see them

You have the subtotal:

[ITEMAMT] => 21.97 It doesn't correspond to the total of all items: [L_AMT0] => 19.97 [L_AMT1] => 0.00 That's the reason why PayPal is showing the error.

Here is how the function should look in the observer:

public function updatePaypalTotal(Varien_Event_Observer $observer)
{
    $cart = $observer->getEvent()->getPaypalCart();

    $donationAmount = $cart->getSalesEntity()->getDonationAmount();
    if ($donationAmount > 0) {
        $cart->addItem('Additional Fees', 1, $donationAmount, 'donation');
    }

    return $this;
}

A little about the registry...

Mage::registry('check_sales_order_before', false);

This is a read operation. It will check if the registry key with check_sales_order_before is set, if so it returns it otherwise null.

To write to the registry, you need to use register():

Mage::register('check_sales_order_before', false);

Please note that by default Magento will throw an exception if you try to register a variable more than once in a session (e.g. if you want to change false to true later on). In your case it's best to utilize the "graceful" argument to ensure it's done silently:

Mage::register('check_sales_order_before', false, true);
// do stuff...
if (...) {
    Mage::register('check_sales_order_before', true, true);
}

Then finally, once your checkout has been completed:

Mage::unregister('check_sales_order_before');

Your implementation

Where do I put the ... code?

Well you don't actually need to initialize this value. Just check if it's set before you do anything, then set it to true once you've done something:

if (Mage::registry('have_added_products_already')) {
    // do nothing, it's already been done...
    return;
}
// do stuff, it hasn't been done yet...
Mage::register('have_added_products_already', true, true);