req. get nested variable in header - node.js

Question

How do I get the SellerId?
req.headers['user-agent'] returns me the User Agent but nesting further with multidimensional arrays doe not bring me forward.
e.g.
console.log(JSON.stringify(req.headers['user-agent']['message']));
returns undefined
POST / HTTP/1.1
x-amz-sns-message-type: Notification
x-amz-sns-message-id: 0000000-0000-00000-0000-00000EXAMPLE
x-amz-sns-topic-arn: arn:aws:sns:EXAMPLE:11111EXAMPLE:TestTopic
x-amz-sns-subscription-arn: arn:aws:sns:EXAMPLE:11111EXAMPLE:TestTopic: EXAMPLE
Content-Length: 999
Content-Type: text/plain; charset=UTF-8
Host: ec2-EXAMPLE.compute-1.amazonaws.com
Connection: Keep-Alive
User-Agent: Amazon Simple Notification Service Agent
{
Type : Notification,
MessageId : xxxxxx-xxxx-xxxx-xxxx-xxxxxEXAMPLE,
TopicArn : arn:aws:sns:EXAMPLE:11111EXAMPLE:TestTopic,
Message :
{
NotificationReferenceId: 1111111-1111-11111-1111-11111EXAMPLE,
NotificationType: PaymentAuthorize,
SellerId: A3PJMS5WFYSRB4,
ReleaseEnvironment: Sandbox,
Version: 2013-01-01,
NotificationData:<?xml version="1.0" encoding="UTF-8"?>
<AuthorizationNotification xmlns="https://mws.amazonservices.com/ipn/OffAmazonPayments/2013-01-01">
<AuthorizationDetails>
<AmazonAuthorizationId>P01-0000000-0000000-000000</AmazonAuthorizationId>
<AuthorizationReferenceId>P01-0000000-0000000-000000</AuthorizationReferenceId>
<AuthorizationAmount>
<Amount>100.0</Amount>
<CurrencyCode>USD</CurrencyCode>
</AuthorizationAmount>
<CapturedAmount>
<Amount>10.0</Amount>
<CurrencyCode>USD</CurrencyCode>
</CapturedAmount>
<AuthorizationFee>
<Amount>1.0</Amount>
<CurrencyCode>USD</CurrencyCode>
</AuthorizationFee>
<IdList />
<CreationTimestamp>2013-01-01T01:01:01.001Z</CreationTimestamp>
<ExpirationTimestamp>2013-01-01T01:01:01.001Z</ExpirationTimestamp>
<AuthorizationStatus>
<State>Completed</State>
<LastUpdateTimestamp>2013-01-01T01:01:01.001Z</LastUpdateTimestamp>
<ReasonCode>None</ReasonCode>
</AuthorizationStatus>
<OrderItemCategories />
<CaptureNow>true</CaptureNow>
<SoftDescriptor>AMZ*softDescriptor</SoftDescriptor>
</AuthorizationDetails>
</AuthorizationNotification>,
Timestamp:2013-01-01T01:01:01Z
}
Timestamp : 2013-01-01T01:01:001Z,
SignatureVersion : 1,
Signature : rkne..9=kOUhF,
SigningCertURL : https://sns.EXAMPLE.amazonaws.com/SimpleNotificationService-aaaaaabbbbbbccccccEXAMPLE.pem,
UnsubscribeURL : https://sns.EXAMPLE.amazonaws.com/?Action=Unsubscribe&SubscriptionArn=arn:aws:sns:EXAMPLE:11111EXAMPLE:TestTopic:GUID
}