I set a fixed price for a product, but this isn't exported to the marketplace, what now?

When your price isn't sent to the marketplace or gets overruled by another price, the chance is big that your priorities are not set up correctly. Maybe the price priorities are configured in a way that your fixed price does not have the highest priority. 

You can configure this in your channel configuration in the price settings tab. Read more about this via this link (go to article 2-D).