Amazon will not. Even if they end up having to use separate boxes, or ship one thing now and another later, you only pay one shipping charge.
If a vendor is fulfilling, they usually follow the same practice but will normally wait until all can be shipped at once and in the same box. The shipping calculation engines are pretty good at this now.
Believe it can vary – I have been charged shipping for each item from the same vendor which I have been disappointed about because the shipping charges were more than the item costs
Not if you have Amazon Prime, or you are buying over the minimum requirement for free shipping. At least it was like that for me when I was buying things off of Amazon the last time I was shopping on their.