how to use an if statement in excel to calculate commissions or flat fee

I am trying to use an IF statement in excel. I would like the statement to do the following: I have 3 fields (C13, D16 & E16) that are user inputs. C13 holds a sales price, and then the user either enters a commission percentage rate (D16) or a flat fee (E16). If either one of these fields (D16 or E16) is NOT blank, I would like to apply a sum (for D16) or display the value entered (for E16) into D18. I hope I stated this clearly. If I have not please let me know.

this is what I have done so far, but obviously I have done something wrong as it is giving me an error.

IF(OR(NOT(ISBLANK(D16) C13*(D16*.01));(not(isblank(E16)=E16))) I am using office 2011 for mac.