I have gotten things from the NARA off and on for years on various subjects (even did some research in the DC archives a long time ago). Most was about early sailing vessels but I also researched a couple Civil War men.
About three years ago I was researching the post office of a small Maine town. I started by "looking around" for what the NARA should have, then my first request was "for all records of ..." and I set an excessively large dollar limit.
Each time something came I would e-mail back that my order had not been completed and they should also have ...
It went on this way until finally a letter came explaining that I "had it all". The total cost to me was very reasonable and far below the limit I set.