This didn't work for me, because it didn't recognise the 0% commission for the first 14K. I used a Sales total of 20K, and it's telling me the commission is $2000, when it should only be $500.
You get paid 7.5% on the $2000 between $14001-$18000, which is $300 for the 18K. You then get paid 10% for revenue over $18001. So say you made $20,000. You would get $300 for the 14-18K, and then $200 for the $18001-$20000. The function is just making it 10% on the whole figure, rather than sectioning off.

Does that make sense?