Dual Pricing for cruise to Greek Islands? (one price (low) for Greeks and another (high) for tourists)

Earlier in the fall my wife spent a month in Athens visiting relatives. FYI, she was born in Greece but is a US naturalized citizen with a US passport. While in Athens, a travel agency was advertising a 3-day weekend cruise to the islands for 374 euros. When she made arrangements to buy a ticket, the travel agency asked for her passport. When they determined

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