Error on settingspage Mixed Content

I’m getting an error when visiting the settings page while using SSL

Mixed Content: The page at ‘mydomain.com/wp-admin/admin.php?page=wpfastestcacheoptions’ was loaded over a secure connection, but contains a form which targets an insecure endpoint ‘http://api.wpfastestcache.net/paypal/buypremium/’. This endpoint should be made available over a secure connection.