Originally Posted by Evan!

1) I started here http://www.hawaiianairlines.com/
2) Hovered over "Book Travel" at the top of the page and clicked on Car Rentals which took me here.
3) Scrolled down to Avis/Find Avis Specials. (<<K816401 is part of the link!)
4) Which took me to avis.com with K816401 pre-filled into the AWD field on the main booking page. (<< because K816401 was part of the link)

Maybe..... you transposed 2 numbers in your reservation.... the agent was looking at the wrong reservation... the agent fat-fingered the code when cross-referencing it with T&C... ???

I'm not saying these any of these things happened but you asked where the info came from that K816401 was for Hawaiian Airlines. This is where I got it.