Since the biggest part of your rental cost is the Underage Fee, you should try to figure out a way to get it waived. The easiest option is to use your AAA / USAA membership, if you have one. If you're not a member, do as [MENTION=264140]Brighton Line[/MENTION] suggested and ask you father/brother to add you to their account. Even with the $30-60 you'll spend on the AAA membership, you'll still come out a winner. You'll also continue to benefit whenever you rent until you're 25. IMO, that's money well spent.
[MENTION=117200]IAHtraveler[/MENTION] summed it up pretty well & I too would advice against using discount codes you're not entitled to use. I'm not sure what Citigroup discount code you're using. Is it for Citi customers or is it for Citi employees? How do you even know it's a Citigroup discount code and not something else? Did you find it on Citigroup's website or on the rental car company's website saying it was a Citigroup discount?