My Dentist Didn't Inform Me About Invisalign Express. I Believe I'm a Candidate. Should I Mention It?

If Express is an option, it can only be done if planned at the start.  More specifically, the lab fee for this is set and is determined by Align technologies, not your dentist.  You were offered a significant reduction in fee already, and in my office the Express is more than what you paid for this full treatment.  It is likely that your doctor is the same, and did not offer the Express as the fee was higher and not worth mentioning.

If you did not sign a contract  you have no responsibility. also if the trays have not been shipped you owe nothig.

Not only would I not mention it;  I would refer as many patients as I could to him as a small token of your appreciation.  You got the discount of a lifetime!  Most dental offices operate at 65-70% overhead making a fee like what you paid a mistake on the part of your dentist.