Answer by ScottG for Rails Peddler gem: Using Amazon MWS Feeds API for adding products to Amazon seller central store

FeedContent is the actual content of the feed itself, whether XML or flat file. You also need a FeedType. Have a look at the MWS Feeds API overview to learn about the process. No matter the language or platform you use, the process is the same.

A request is going to look like this:

POST /Feeds/2009-01-01 HTTP/1.1
Content-Type: x-www-form-urlencoded
Host: mws.amazonservices.com
User-Agent: <Your User Agent Header>

?AWSAccessKeyId=0PB842ExampleN4ZTR2
&Action=SubmitFeed
&FeedType=_POST_PRODUCT_DATA_
&MWSAuthToken=amzn.mws.4ea38b7b-f563-7709-4bae-87aeaEXAMPLE
&MarketplaceIdList.Id.1=ATVExampleDER
&SellerId=A1XExample5E6
&ContentMD5Value=ExampleMd5HashOfHttpBodyAsPerRfc2616Example
&SignatureMethod=HmacSHA256
&SignatureVersion=2
&Timestamp=2009-01-26T23%3A51%3A31.315Z
&Version=2009-01-01
&Signature=SvSExamplefZpSignaturex2cs%3D

and the body, which contain the products you are adding looks like this:

<?xml version="1.0" encoding="iso-8859-1"?>
<AmazonEnvelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:noNamespaceSchemaLocation="amzn-envelope.xsd">
  <Header>
    <DocumentVersion>1.01</DocumentVersion>
    <MerchantIdentifier>M_EXAMPLE_123456</MerchantIdentifier>
  </Header>
  <MessageType>Product</MessageType>
  <PurgeAndReplace>false</PurgeAndReplace>
  <Message>
    <MessageID>1</MessageID>
    <OperationType>Update</OperationType>
    <Product>
      <SKU>56789</SKU>
      <StandardProductID>
        <Type>ASIN</Type>
        <Value>B0EXAMPLEG</Value>
      </StandardProductID>
      <ProductTaxCode>A_GEN_NOTAX</ProductTaxCode>
      <DescriptionData>
        <Title>Example Product Title</Title>
        <Brand>Example Product Brand</Brand>
        <Description>This is an example product description.</Description>
        <BulletPoint>Example Bullet Point 1</BulletPoint>
        <BulletPoint>Example Bullet Point 2</BulletPoint>
        <MSRP currency="USD">25.19</MSRP>
        <Manufacturer>Example Product Manufacturer</Manufacturer>
        <ItemType>example-item-type</ItemType>
      </DescriptionData>
      <ProductData>
        <Health>
          <ProductType>
            <HealthMisc>
              <Ingredients>Example Ingredients</Ingredients>
              <Directions>Example Directions</Directions>
            </HealthMisc>
          </ProductType>
        </Health>
      </ProductData>
    </Product>
  </Message>
</AmazonEnvelope>