Customers demonstrate their interest in an item by putting it in their shopping cart. Showing customers similar items to what they already have in their cart is a good way to spur add-on sales.

The CartSimilarities response group, which works with most cart operations, returns items that are similar to the ones included in the cart request. Items are returned based on:

Each of these similarity types have their own element tag: <SimilarProduct>, <SimilarViewedProduct>, and <OtherCategoriesSimilarProduct>, respectively, as shown.

http://ecs.amazonaws.com/onca/xml?
Service=AWSECommerceService&
AWSAccessKeyId=[AWS Access Key ID]&
AssociateId=ws&
Operation=CartCreate&
Item.1.ASIN=B000062TU1&
MergeCart=True&
Item.1.Quantity=2&
ResponseGroup=CartSimilarities

This request produces the following response snippet.

<SimilarProducts>
  <SimilarProduct>
    <ASIN>B00008DDXC</ASIN>
    <Title>Harry Potter and the Chamber of Secrets (Widescreen Edition)(Harry Potter 2)</Title>
  </SimilarProduct>
</SimilarProducts>
<SimilarViewedProducts>
  <SimilarViewedProduct>
    <ASIN>B000E6UZZK</ASIN>
    <Title>Harry Potter Years 1-4 (Harry Potter and the Sorcerer's Stone /Chamber of Secrets / Prisoner of Azkaban / Goblet of Fire) (Widescreen Edition)
    </Title>
  </SimilarViewedProduct>
</SimilarViewedProducts>
<OtherCategoriesSimilarProducts>
  <OtherCategoriesSimilarProduct>
    <ASIN>0590353403</ASIN>
    <Title>Harry Potter and the Sorcerer's Stone (Book 1)</Title>
  </OtherCategoriesSimilarProduct>
</OtherCategoriesSimilarProducts>

This response shows that the Harry Potter and the Sorcerer's Stone DVD placed in the shopping cart is similar to another Harry Potter DVD, Harry Potter and the Chamber of Secrets. Customers who viewed the DVD in the shopping cart also viewed the DVD, Harry Potter Years.. A related item to the DVD in the cart but in a different product category is the book version of the DVD: Harry Potter and the Sorcerer's Stone.