Assuming that you're using a standard "buy now" or "add to cart" button... Apparently, to create a test transaction the PayPal payment form must be directed to the sandbox testing site. I did not find this in the developer documentation. I hacked it myself on a hunch and it worked. In the PayPal button code change: <form target="paypal" action="https://www.paypal.com/cgi-bin/webscr" method="post"> to: <form target="paypal" action="https://www.sandbox.paypal.com/cgi-bin/webscr" method="post"> Adding the "sandbox subdomain allowed the test purchases to complete.
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