in the WA_eCart/PP_ECO_Scripts/PP_ECO_PHP.php file, change line 133:
$retVal = WA_PP_ECO_CallSOAP($postXML, $usesandbox);
to:
$usesandbox = (isset($varvalueArray["usesandbox"]) && $varvalueArray["usesandbox"] == "true");
$retVal = WA_PP_ECO_CallSOAP($postXML, $usesandbox);