Wishing to express sympathy.

[Wet Knight]

Good, better, best. 

Some things can be good and some things can be better than something else, but only one thing can be best.

From this, it follows that whereas one could offer several of one's better wishes to a friend, one can only offer a single best wish.

 

Somewhat the same thing is true of condolence.

A person who has received letters of condolence from friends, has received condolences, but only a single condolence from each friend.

The English language does not permit you to have a theoretical box, of a black edged condolence, ones with a verse and others you have been given but never used, that you can post off as a parcel of condolences.

Because a condolence is singular,  the only way that a person could "Send their condolences", would be to send a number of separate condolence, and that opens a can of worms.

I wonder how many, other people are sending.  Would three be enough?, Hmm, don't want to seem mean.  How about six, no eight, you need to be well remembered in their Will;  Hang on, better see how many stamps I've got...............................