American Express Cancel on a R175 $5 Documentary

AM. EX. CO.

MAR  22  1900

David Thompson sends in this scan of an R175 with an American Express cancel.  AMEX cancels are common as printed or handstamped cancels on the 1 cent documentary; an AMEX cancel on a dollar value is seldom seen.

What business did AMEX have in using a $5 documentary stamp?  I can't imagine its use to pay for 500 entries in a shipping ledger.  If you have an answer, and especially if you have seen a dollar value documentary stamp on an AMEX document, please write to 1898revenues@gmail.com.