Who can I contact about very poor customer service?

The Get Support link won't help you, your best bet is to contact the Store Manager and file your complaint there. If this was a Genius Bar situation you might try asking for the Service Manager instead. Be sure to have the date and time at hand so the manager can check video if applicable and be specific regarding your complaint. If all they did was follow the rules you won't get far.

You can also call AppleCare and ask to be transferred to Customer Service but that is the long way around. If you are not satisfied with the manager's response you can ask for the name/contact information of their supervisor.

Unless you asked to be transferred to Customer Service calling AppleCare and reporting the incident will do nothing. I didn't say ask for a supervisor, call the store and ask to speak to the manager. That will have far more impact than writing a letter. Did these people verbally attack you, curse at you, or something like that?

You are not talking with Apple you are talking with other users.

Click the link above "Get Support" and go from there.