While trying to align your document's merge fields with the fields that are coming over from a 3rd party integration, you can use "Debug" mode. Debug mode will save merge data (for 24 hours) so you can analyze what is being sent to merge and determine if you are using the correct merge field codes.
Here's how to turn on and use Debug Mode:
1. Go to the Settings tab for your Document or Data Route
2. At the bottom, turn on Test mode, then check the box to save the data.
3. Go submit another merge from your 3rd party integration
4. Go to the Overview tab for your Document or Data Route and you'll see a "View Data" button in the Recent Merges section. Click that and you'll see the data that came through with the merge. You can even click on the name of the field and it will show you the merge field codes to use.
If you don't wish to place your document in Test mode, alternatively, you can turn on the 'Save Data to Re-Merge/Download Later' option under Settings > Advanced Settings.