Aries:
Shulchan Aruch disagrees with your points.
There is no difference between shadchanim and any other services provided. If you would like i will qoute you chapter and verse from the shuldchan aruch.
here’s a start (i just pulled this off a website called Businesshalacha.com and did a search using the word shadchanus)
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Paying a dollar certainly does NOT fulfill the halchachic obligation any more that it would if a electircian came by to fix your light fixture and didn’t qoute a price. Does that depend on BP or FR. 13th ave or Flatbush. That is certainly a legitimate question that should be answered by a competent posek in these matters. It is not specifically related to shadchanus anymore than it is realted to any other service provider who didn’t make a clear agreement prior to providing the service.
Regarding paying before the chassuna or afterwards, with my deepest sympathies to “bad omens” they generally don’t consitute codified halacha. If one would like to do so, they by all means may. However unless that is the community custom it can not be enforced. If a shadchan would demand payment before the chassuna but no clear custom has been established, then the chosson/Kallah sides would be (to the best of my understnading) in their halachic right, for waiting until after the chassuna to pay.
This actually has potential ramifications for scenarios when a shidduch breaks off. Look in shulchan and commentators where this is disucssed.
This all being said, For what it’s worth I personally do think it is apporpriate to give the shadchan commesurate shadchanus shortly after the engagment if not at the actaul engagment party. Nothing to do with bad omens, but everything to do with the proper way to express ones deepest appreciation for bringing themsleves or their child to this wonderful stage in life.