Presumably what their fee covers is their work arranging it. I don't think it makes a whole lot of sense to charge much more for ordering a plane ticket than a pizza; it's not so much more effort that a percentage makes much sense. On the other hand, "get me a Philly cheesesteak delivered" (which someone mentioned) may entail both "find me a cheesesteak" and "find me a courier" - so you charge more because it's more effort.