The FCC definition of a kit (from Part 15.3(p)) is:
(p) Kit. Any number of electronic parts, usually provided with a schematic diagram or printed circuit board, which, when assembled in accordance with instructions, results in a device subject to the regulations in this part, even if additional parts of any type are required to complete assembly.
There have been some rulings that simply plugging ICs into sockets and/or mounting an assemble board into a cabinet does not constitute a kit. The assumption is that some level of electronics expertise is required to construct a kit. That makes sense to me. I guarantee you that when you see the bag of parts in the Eggfinder, you will know that it is truly a kit by anybody's definition. It will be a little more challenging than an Eggtimer, but I'm including some very fine low-temperature solder to make mounting the GPS module and the MCU easier.
The FCC reference is from the aforementioned enforcement action, the opinion was written by John Poutasse who was and still is the Cheif of the Spectrum Enforcement Division. Contained within it, and I quote:
Section 302(b) of the Act provides that "[n]o person shall
manufacture, import, sell, offer for sale, or ship devices or home
electronic equipment and systems, or use devices, which fail to comply
with regulations promulgated pursuant to this section." Under Section
2.803(a) of the Rules, the Commission prohibits the marketing of radio
frequency devices in the United States unless the devices are
authorized under the appropriate equipment authorization procedure and
comply with the applicable technical standards as well as the
administrative requirements relating to equipment labeling and
consumer disclosure. Under Section 15.201(b) of the Rules, intentional
radiators, such as AM transmitters, must be certified in accordance
with the procedures set forth in Sections 2.1031 through 2.1060 of the
Rules. "Kits," as defined in Section 15.3(p) of the Rules, however,
generally do not require authorization.
This pretty much lays it out... if you produce a KIT, you do NOT need to go through the lengthy and expensive certification procedure. There is an exception for "TV interface devices"; it goes back to 1989. I think that's about the time that a lot of kit cable descramblers came out, and a lot of them were very poorly designed. An engineer friend of mine got one, it barely worked and it screwed up the other TVs in his house so he quit using it. My guess is that the FCC got a lot of complaints from neighbors and felt compelled to put a rule in place. Back then, there was virtually no satellite TV, and cable TV penetration was maybe 50%, so anything that leaked TV RF had a good chance of screwing up TV reception.