Grumman Aerospace sent North American Rockwell an invoice for towing the Apollo 13 Command Module back with the Lunar Module.
In response North American reminded Grumman that North American Rockwell had not received any payment for ferrying the Lunar Modules on previous trips to the moon.
Edit The picture seems to be blurry on lemmy here is a link of a higher resolution picture Link
I think theres an implicit understanding that if you dont pay, then its no longer confidential