Most important, the services of a shadchan are optional. No one forces you to use them. You can make a shidduch yourself, if you like.
I understand the point of the thread, though.
If you use the services of a professional shadchan, you are creating an implicit contract, and the shadchan deserves to be paid.
I think the question was originally asked because the shadchan’s fees are not posted. This too, is unfair. I’ve always disliked people telling me, “oh, just pay me what you think is fair.” I prefer the vendor to tell me his price, then I can choose to accept, negotiate, or go elsewhere.
If a payment is required for honest labor, which this is, please tell me in advance what that fee will be. I will not think less of you.