so is the complaint that you had a delay or that the supervisor bent the rules and their policy and only charged you $300 for the change. for it being a mistake that you admitted to being your fault, i say you got off pretty good compared to what some others have had to pay
Reservations- <LI class=MsoNormal style="MARGIN: 0in 0in 0pt; tab-stops: list .5in; mso-list: l6 level1 lfo6; mso-margin-top-alt: auto; mso-margin-bottom-alt: auto">Reservations are subject to cancellation if you are not available for boarding, at the boarding gate, at least 20 minutes between U.S. points, to/from Canada, Mexico, or the Caribbean; 30 minutes to/from Hawaii; and 30 minutes to/from international points;
- Please cancel your reservation if there is a change in your travel plans. Failure to cancel a confirmed booking may result in automatic cancellation of your entire itinerary.
its in the contract of carriage on their website