You could always create a document and label it as something else, write the credentials in there and then rename the document to something else, so that it would appear as an .exe or .avi.
And then just make sure the guy who's supposed to receive the credentials knows that once he's received the for instance .avi knows he's supposed to rename it back to .docx, that way someone "listening in", wouldn't know what to do with it.
I think anyone with the skill and ability to be listening will be wise to that kind of ruse. If you trust the person on the other end, I would go ahead - what are the chances of your being targeted for the theft?
Tbh I doubt it, unless they're really going through everything you send they wouldn't find it I think.
Anyway, like you say, the chances are soo slim that it is NOTHING to worry about.