A2*C2 is a product. Why would you want to sum a product?
In your example the SUM function is not necessary
try =If(A2,A2*C2,B2*C2)
As you have posted in the programming section, do you need a VBA solution?

__________________
Using O365 v2503 - Did you know you can thank someone who helped you? Click on the tiny scale in the right upper hand corner of your helper's post