In the Green P lots you are required to pay. However, they expect that you will not leave the pay-and-display ticket on the bike for another motorist to use. When they leave the "non-payment" envelope on your bike you simply tear off the portion of the receipt that shows the date and time you paid and put that in the envelope, then deposit the envelope in the box near the ticket dispensing machine. Keep the receipt portion just in case they come back and say you did not pay and they did not receive the proof of payment in the box.
I have sent e-mails to the President of Green P attempting to explain how this system is flawed and that you might not know that the lot was checked and a "non-payment" envelope was left on your bike, because it could blow away or be taken by others. If these things happen, you would not know that your plate number has been noted and would not put the proof of payment in their box. So you have to hang onto the receipt until you renew your plate, just in case they try to say you have outstanding parking fines. I suggested that the corner spaces, which are usless for cars be reserved for bikes for free, but that was flatly refused. My suggestion is to avoid unmanned Green P lots unless there is no other choice.
