Most Americans that I have pointed this out to look at me in disbelief. The vast majority have never heard of US Customary Units. Whenever they refer to their system of measure they say imperial, and many are unaware that volume measure in the Imperial system is different from US Customary. I fought again the use of imperial as the name for the system for over twenty years in a multinational company to no avail.
Send them this handbook from NIST! :) Section 2 covers all of the customary measurements, some with names hardly anyone knows (Apothecaries Units? Gunter's Chain Units?)