Cost analysis

Why is it common to assume a fixed sales mix before finding the break-even volume with multiple products? How do income taxes affect the break-even equation? Why? What are some of the important assumptions commonly made in cost-value-profit (CVP) analysis? Do these assumptions impose serious limitations on the analysis? Why or why not? In a decision to open a new sales channel (e.g., online sales), what are some examples of the differential costs? What costs would not be differential?