Shekalim perek 1 - Kalbon (קלבון)

I am having trouble following the discussion of the kalbon, the extra small coin that people need to pay along with their half-shekel. The Rambam says that during the time of collecting machtzis hashekel, half-shekel coins are in demand and full-shekel coins are not.  So I guess the relative prices shift a little, and the money-changers might require a little bit of encouragement to change full shekels into half shekels. What I don't understand is, why does this require a gezeirah? Surely the money-changers know their business, and will charge whatever they need to charge. And if one charges too much, he won't get customers - same as any other free market situation. Why are rules required for this? And such detailed rules - some people are doing a bigger mitzvah, or more self-sacrificing, like women who are giving when they don't have to, or anyone giving on behalf of someone else -- and then such people don't need to give the kalbon. Again, how is that going to work? It sounds like the money-changer market isn't what's determining this at all.

Update: I learned a little more about this, and I'm a little more comfortable. Some good places to look are: 1) Rashi in several places in Shas where this mishnah is brought, the Rambam perek 2 of Shekalim (in Sefer Z'manim), and the Meiri on the mishnah.

The Meiri suggests that the shulchanim money-changers generally themselves deliver the shekalim to hekdesh.

Rashi says that the shekalim are often under-weight, or are small and get lost, so we add the kalbon to make sure that people give a full half-shekel. According to him, you have to give a kalbon, as least  d'rabanon, even if you give just a single half-shekel.

According to the Rambam, the kalbon is what you'd have to give anyhow to a shulkhani if you tried to change in a shekel for a half-shekel. The reason is that the demand for the half-shekel goes way up this time of year, and becomes higher than half the price of a full shekel. Chazal made that an actual takanah so that the shulkhani will willingly give a full high-quality shekel to hekdesh on your behalf (Meiri). They even were mesaken that you need to give a kalbon if you try to circumvent the shulkhani by giving your shekel straight to hekdesh instead of the shulkhani.
As a result, you will never have to give a kalbon unless two of you are giving a full shekel together, or one of you gives a full shekel and asks for change. (R' Meir maybe disagrees with that, depending on your girsa.)