Can extra HTTP headers influence on MWS Feed API submission?

It is nasty to debug Amazon MWS Feed submissions because whatever you might be doing wrong Ama just returns the same senseless error message:

<Error>
  <Type>Sender</Type>
  <Code>InvalidParameterValue</Code>
  <Message>Either Action or Operation query parameter must be present.</Message>
</Error>

I am 100% positive that I build StringToSign correctly as well as calculate HMAC-SHA256, etc. It took me a few weeks to collect and adopt a stack of useful functions for hashing, signing, base64ing, etc. MWS requests. Written in pure Pascal they were all tested on Orders and Products APIs.

Now when it comes to Feeds API I am stuck on the above error. All parameters are equal to those generated by MWS Scratchpad. I tested the submission StringToSign generated by MWS Scratchpad, but no luck.

What I noticed so far: there is a difference between the number/values of headers generated by MWS Scratchpad and my app.

The Scratchpad generates the following headers (it least they are displayed):

  Host: mws.amazonservices.ca
  x-amazon-user-agent: AmazonJavascriptScratchpad/1.0 (Language=Javascript)
  Content-Type: text/xml

My app uses Indy (in XE4) TIdHTTP to make a request. When Amazon returns the above error, Request.RawHeaders.Text contains the following:

  Content-Length: 251
  x-amazon-user-agent: MyApp/1.1(Language=Delphi;Platform=Windows7)
  Content-Type: text/xml
  Host: mws.amazonservices.ca
  Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
  Accept-Encoding: identity
  User-Agent: MyApp/1.1(Language=Delphi;Platform=Windows7)

Looks like the extra the headers are added to Request object by default. My first question is: can those extra headers theoretically influence on the behavior? I.e. can they be the showstoppers?

My second question: is there any option to mofidy the list of default heades in Indy HTTP.Request? To go on with the debugging I would rather exclude extra headers to see if the request works.