Refund of Unused Value on SPTC (Shanghai Public Transportation Card)

I am leaving the city on 23 June in the early morning. I would like to get a refund of the unused amount on my card. I'm guessing it would be about 60-65 RMB. I see in the notes that an invoice has to be submitted. How and where do I get one? I speak very little Chinese. Also, I had hoped to take care of this on Monday 22 June but it will be the Dragon Boat Festival between 20-22 June. Will any of the refund locations be open then or should I go to one on Friday?