Paul the cab driver is such a minor character, he shouldn't even deserve a mention. However, he delivers one of the book's most awesome phrases. When Llewelyn tells him, "[T]rust me," Paul doesn't like it. Llewelyn asks if he's ever said those words himself, and Paul responds, "Yeah. I've said em. That's how come I know what they're worth" (7.2.107).
In other words, if someone tells you to trust them, they're going to let you down. And although Paul responds unscathed, Llewelyn lets basically everyone else down. Or he gets them killed, which is the ultimate letdown.