Aagh! Nosferatu! What a film. All those coffins of earth.
Unfortunately (?) fuller's earth is a lot more prosaic than a resting place for the Transylvanian count and his mob.
It's a soft, friable clay that absorbs grease and was used in the textile industry to clean, bleach and scour cloth, particularly woollen cloth. It assisted the fullers in, er, fulling the cloth, if you will. (Well, how boring can you get?) Another use is in refining fats and oils, including petrol.
As to why American Express were importing it? Maybe for its use as a 'filler' (not fuller!) in papermaking. It's also used as a coating for printed papers. Fuller's earth was (is?) mined in England and other parts of the world, but also in the US. Maybe it was a particular sort/grade of fuller's earth?
My grandma was still using it as a household cleaner in the 1980s - and there were no American Express cheques (damn!) or vampires in her house.
On a lighter note, it's also the name of a hysterical science show spoof Peter Sellers did back in the late 1960s/early 1970s.
Ok, that's probably too much information.
Cheers,
F