Why do I see a tick mark with 'Done' on my iPhone when the terminal in the shop says the payment has been declined?

You will see a tick mark with 'Done' message on the device regardless of whether the transaction has been successful or declined", since it only indicates the communication between the digital card and terminal is successful, and the information is sent to issuer. The transaction might be declined by the issuer if there is not enough balance, or the transaction was recognised as fraud.