Please allow the PayPal Developer sandbox as a URL in the admin settings:

SIMPLE_PAYPAL_URL_SANDBOX => 'https://www.sandbox.paypal.com/'

Without this support it makes it difficult to test modules using the Simple PayPal system.

The additional 'test' URL needs to be checked in the simple_paypal_ipn_verify() function too, in order to provide debug in the watchdog log.