The newest version of Magento in some cases has a problem working correctly with PayPal Standard payment option. After completing the checkout fields and clicking the buttons to process payment via PayPal, you can experience the error message ‘Unable to process payment.’ There is a trick to solve this issue.
The error message looks like this:
Unable to process payment. Please contact the merchant as the shipping address provided by the merchant is invalid, and the merchant has requested that your order must be shipped to that address.
Solution (confirmed with PayPal technical support and tested by Royal Deer Design):
The error that your are receiving is associated with a variable that your Magento shopping cart is likely sending to PayPal. You have to locate the following line of code in your Magento shopping cart: . It is located in app/code/core/Mage/Paypal/Model/api/Standard.php.
This line of code should be removed, commented out, or the value should at least be changed to 0.