We now provided the travel agent with the precise name of the promotion, and she called Princess. Princess was unable to locate a lower price.
Now I've sent the agent a screen shot of the lower price.
Our call to Princess went nowhere; as we have an existing TA booking, Princess will not discuss price with the customer, only with the TA.
I took great delight at agreeing to answer a customer survey after the call, and ranked Princess 1, on a scale of 1 to 5, in terms of customer service.