You are using the Soap service in the wrong way.
If you look at the example on the page for the call "GetProductByProductNumberInformation" on http://webservice.allposters.com/ProductInformationService.asmx?op=GetProductByProductNumberInformation there is only a placeholder "string" mentioned, but you are sending a complete set of XML. This is probably wrong.
I don't know why you think you can send more than a string like the XML you did, but I found out that this service actually expects you to send your XML wrapped inside a CDATA so that it is just a string - the server then unpacks the string and does another XML parsing.
This implementation method is completely bullshit, because it circumvents the point of having a Soap Service with a WSDL description of what kind of parameters the service allows and expects - but you are most unlikely to change that.
So you have to make NuSoap to wrap your XML string inside CDATA tags, otherwise it won't work at all, I think.