I would think you are one of thousands, since PbP parking came in ! Councils have a duty to behave fairly and promptly and in the public interest. They have lost no money from your mistake. I use RinGo up here in Crewe and Nantwich and have five cars listed, so I too, have to be careful ! So far so good.
So you need to compose a non-confrontational representation explaining what happened and asking, in view of you making payment, not avoiding it, that they cancel the PCN. Nothing ventured, nothing gained, and if they reject your reps, they'll re-offer the discount.