I've been struggling with this one all day. I've taken the c# code from the example code as well as using a few other tutorials but the IPN simulator always gives me invalid. I've checked that it is returning the data exactly the same as i am receiving (except for the cmd variable added to the beginning). I've attempted to do this 4 different ways now but its still giving me the invalid error every time. 

Im currently using the ipnpb.sandbox.paypal.com/cgi-bin/webscr URL. My development site is running over https with TLS1.2. I'm sending the received data back as is with the added cmd=_notify-validate variable and have confirmed they are in the same order etc. Is there any possible way of getting more information out of this when I'm simulating as INVALID is a really unhelpful error?