When shopping on Amazon, there are some relatively cost-effective categories of goods:

1. Daily necessities: such as paper towels, toothbrushes, shower gel, etc. The prices of these products are relatively stable in supermarkets or physical stores, but there may be more affordable promotions on Amazon.

2. Electronic devices: There are a wide variety of electronic devices on Amazon, such as Kindle e-books, Echo smart speakers, Fire tablets, etc., and there are often promotions and discounts.

3. Clothing: There are many branded clothing on Amazon, and there are often discounts. In the US, Amazon's clothing delivery and returns are very convenient, which can give consumers more choices.

4. Food and beverages: On Amazon, some specialty foods such as imported snacks, health products, and alcohol are usually cheaper than physical stores such as supermarkets, which can also improve the shopping experience.

5. Other small items: For some small items in daily life, such as compression bags, storage boxes, car chargers, etc., Amazon's prices are often more competitive than those in physical stores.

In short, when shopping on Amazon, you can find the most cost-effective product categories by comparing prices and searching for shops with good reputations. However, you need to pay attention to the quality and authenticity of the products to ensure that the quality of the purchased products is guaranteed.