The questions at hand is: are we hiring an advertising agency to display advertisements or are we simply buying/leasing billboard space? Is it based on the labor or time and effort of the advertising agency or the scope of the whole project?
FAR part 2 Definition of Supplies: “Supplies” means all property except land or interest in land. It includes (but is not limited to) public works, buildings, and facilities; ships, floating equipment, and vessels of every character, type, and description, together with parts and accessories; aircraft and aircraft parts, accessories, and equipment; machine tools; and the alteration or installation of any of the foregoing.
You are not buying a billboard or a specific advertisement (in other words not paying for the production of a video that the Government would own). Instead, you buying the services of an ad agency to display ads. See FAR 5.504 which refers to services.You are not buying a billboard because you don't own it. You are not buying a billboard space that would allow you to put up any ad you want- like a "Go Army/Beat Navy" ad. You are hiring someone to display ads- that is a service as described above.
FAR 5.504 Use of advertising agencies.
Basic ordering agreements may be placed with advertising agencies for assistance in producing and placing advertisements when a significant number will be placed in several publications and in national media. Services of advertising agencies include, but are not limited to, counseling as to selection of the media for placement of the advertisement, contacting the media in the interest of the Government, placing orders, selecting and ordering typography, copywriting, and preparing rough layouts.