Why did I get the error "Unable to process payment. Please contact the merchant as the shipping address provided by the merchant is invalid..."?

Issue
PayPal couldn't process a payment request due to an invalid shipping address.

Cause
Your shopping cart or payment request includes the variable Address_Override=1. This tells PayPal not to allow buyers to edit their shipping address on the PayPal review page.

If the address isn't valid, they'll receive the following error: “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
Remove the Address_Override variable from your payment request.

The document HTML Form Basics for PayPal Payments Standard has further information on the Address_Override variable.