That last part of your formula *IF(I9=5,1,1) is irrelevant.
It says that if I9=5 then 1 and if not then 1..so no matter what is in I9 it multiplies the existing number *1...therefore does nothing.

This shorter formula might work for you...but it will only handle 0-5. If there is a 6 it will fail.
=IF(I5>12,0,I8*CHOOSE(I9+1,0,0.1,0.25,0.55,0.7,1))

Otherwise, your existing formula could be:
=I8*IF(OR(I5>12,I9=0),0,IF(I9=1,0.1,IF(I9=2,0.25,IF(I9=3,0.55,IF(I9=4,0.7,1)))))

or

=IFERROR(IF(I5>12,0,I8*CHOOSE(I9+1,0,0.1,0.25,0.55,0.7,1)),1)