There is a combination of several scarcity factors which make this card a gem.
While all of the international airmail postal card rates are scarce for finding commercial usages, the first few (11, 13, 15) are among the toughest. The S66c domestic postal card is tough to find domestically used commercially, since it didn't have a long life as compared with the S66 issue as a whole. Many, many of the non-flourescent cards were probably still available when it was issued, so it is even less likely that businesses would use this variety. Being used internationally is a great big plus!
This one is used here by a university researcher to request a publication. It's interesting that many of the surviving uprated domestic postal cards were for similar medical purposes.
Place mouse on image to see the reverse side of the card.