From what you're saying you were truly overcharged, you authorized the payment of $300 and instead $1600 was charged. Therefore, if you're not still within the 24 hour cancellation period you would be within your rights to charge back the transaction.

Since you say EVA has a cheaper and better itinerary, my personal suggestion would be to refund your AC ticket, book the EVA flight and request AC to reissue the eCoupon (which wasn't used anyway) with a longer expiration date.