Now Use Custom Fields with Checkout by Amazon

Checkout by Amazon now provides a way for the merchants and integrators to include custom attributes in the order request XML. These attributes can be associated either with line-items or orders.

When we receive your order request that contains custom attributes, we store it, and return it to you as part of the Callback API, or as part of the Instant Order Processing Notification API. Merchants and Integrators can use these attributes to track affiliate purchases, additional order details, custom item attributes, cart id, user id and any other information they need to track.        

Here's our detailed help content for using custom fields
 
Here’s an example code using custom fields:

<?xml version="1.0" encoding="UTF-8"?> <Order xmlns="http://payments.amazon.com/checkout/2008-11-30/">   <Cart>      <Items>         <Item>            <SKU>JKL909</SKU>            <MerchantId>AEIOU1234AEIOU</MerchantId>            <Title>Calvin and Hobbes Reliquary</Title>            <Price>               <Amount>29.99</Amount>               <CurrencyCode>USD</CurrencyCode>            </Price>            <Quantity>1</Quantity>            <Weight>               <Amount>8.5</Amount>               <Unit>lb</Unit>            </Weight>            <Category>Books</Category>           <ItemCustomData>               <CustomText>                  <Text>Charles River</Text>                  <Color>Malachite</Color>                  <StyleID>10090</StyleID>               </CustomText>            </ItemCustomData>          </Item>      </Items>      <CartCustomData>         <CartNumber>0525470948</CartNumber>      </CartCustomData>   </Cart>  ... </Order>