Can you please give me a passage in the book that talks about, illustrates, or exhibits a postcolonial view or a feminist view? (Or an eco-critical viewpoint or a new materialist viewpoint)

In Sea of Poppies, the author uses a postcolonial perspective overall in his examination of the impact of British imperialism on China and India. The British involvement in China was concentrated in the opium trade, which had a devastating effect on many people’s lives even as it provided a livelihood. In India, a British colony administered through indirect rule, income from the opium trade helped support the apparatus of state and enabled the continued domination of a small, distant European country over a much larger one.
In China, growing opium poppies, in a “sea” of flowers hundreds of miles distant from the ocean, comes to take the place of agriculture for food crops. The demand for opium pushes all other crops aside. Ghosh effectively describes how the trade distorted traditional patterns of poppy growing. People had formerly grown only a small number of poppies because the work was extremely labor-intensive. If there was a bit remaining beyond what each family needed, they would sell that. They did not grow poppies or make opium for the express purpose of sale. It was not seen as a good investment of time and resources.
With the increased trade, farmers shifted to mono-cropping: the poppies were the only thing they cultivated. The corresponding labor requirements were increasingly arduous. The English employed agents who went around contracting people to plant; it was almost impossible to refuse the cash advances they offered. When the crop yields were not as high as expected, however, the farmers could barely repay these advances, or not at all, and thus became indebted to the system. They could not fall back on their own stores because they were not growing food crops.

[T]he English sahibs would allow little else to be planted; their agents would go from home to home, forcing cash advances on the farmers, making them sign /asámi/ contracts. It was impossible to say no to them: if you refused they would leave their silver hidden in your house, or throw it through a window. It was no use telling the white magistrate that you hadn't accepted the money and your thumbprint was forged: he earned commissions on the opium and would never let you off. And, at the end of it, your earnings would come to no more than three-and-a-half sicca rupees, just about enough to pay off your advance.”

In the process Ghosh describes, not only is a harmful addictive substance encouraged, but the entire economic base of the society is undermined. Ghosh’s analysis of the functioning of colonial economy comes from his postcolonial perspective on the social impact of this type of political economy.