If it's just cosmetic, just get some epoxy and glue the chipped piece back on?
I think that's what I would do. I doubt a repair shop could get a new housing and transfer everything into it
This is why cameras with Magnesium bodies cost more, and why pros tend to use them
From what height did it fall?
When Sony bought out Konica-Minolta, part of the agreement was for Sony to use Minolta's proprietary hotshoe for x amount for years. Now Sony is changing to ISO standard hotshoe, but its not yet deployed across their whole product range.
Why is this only on my A99 and not the A65 that I had?
Ouch! Like others said, the repair would be prohibitively expensive. I would check if its still perfect functionally and use it as-is. How high did it fall from? Did it hit concrete, ground, etc?
The A99 is a magnesium alloy body.
That is quite a bold statement. IMHO, all present cameras have plastic bits that can break, and the "pro" magnesium bodies may not break as easily, but it can dent.The Sony cameras use a Magnesium Alloy "body" (internal frame), but are made of plastic housings... A bit like Canon's 5d series.
It's a smart marketing ploy to make it sound as robust as a D800 or 1D, but it's false, in the end.
http://www.eoshd.com/wp-content/uploads/2012/10/DSC03229.jpg