Am I responsible for a charity donation which has later been fly tipped?

I made a donation of, amongst other items, a ceramic ornament in a padded envelope to a charity shop.  A week later, I received a fixed penalty notice for fly tipping as my name and address on the envelope had been found dumped near the charity shop, along with other items which I did not donate.  I have contested this with the council, saying that I did not commit the act of fly tipping, and that the envelope with my name/address on was donated to and accepted by the charity shop. They rejected my explanation and continue to demand payment.  Am I responsible for this envelope and on what grounds can I contest their accusation? The envelope is their only link to me.