What does Nat tell Kit about their cargo? Why do you think he has "biting mockery" in his voice?

Nat tells Kit that he is transporting windows for the house William is building. I believe his tone is mocking because he is jealous.... and also because she didn't tell him herself.

"An interesting cargo we had this trip. One item in particular. Sixteen diamond-paned windows ordered from England by one
William Ashby. They say he's building a house for his bride. A hoity-toity young lady from Barbados, I hear, and the best is
none too good for her. No oiled paper in her windows, no in- deed!"
She was taken aback by the biting mockery in his voice.