POST_FLAT_FILE_FULFILLMENT_DATA in JSON

[Lemodo20]

So far, I have been using the Fika API to transfer various data such as stock levels, price updates and tracking data to Amazon. These are either CSV data or XML (e.g. for tracking data). Since this is to be switched off in March (I understand that CSV data will no longer be possible), I am now looking for a way to do this via JSON. I have already created a corresponding function for generating JSON. However, I am currently failing with the corresponding transfer calls. I've tried several, but either an XML is expected or Amazon issues an error message. For the tracking data in XML I have so far used POST_FLAT_FILE_FULFILLMENT_DATA. Is there an equivalent for JSon? JSON data does not accept this despite the ContentType.JSON parameter.

[Lemodo20]

so nobody have to send json-files to amazon?