You should state your case and exaggerate it, to the manager, supervisor, proprietor, as that discussion heats up, you should play a trump card, for gaining an edge, in public service Theres an unwritten rule, the customer is ALWAYS RIGHT, even if they are wrong, their right, to any and all providing service, choose your words carefully, play it out in your head, ahead of time, tweak it, to make it seem ludicrous that they would even ever expect anyone to pay, under the given circumstances, make it sound ridiculous that they expect payment, flip the script, ask them if they would be happy, if you charged them for a service that wasn't used, or was purchased by them for you, you have to make them believe what you feel and believe, see it your way, as it's the only proper, appropriate way, stick to the statement, the customer is always right, timing of all aspects and trump cards played, is key to winning the debate.

Good luck comrade.