I'm no lawyer, but have been thru the FCC testing process a few times. For a ordinary device that doesn't deliberately transmit (called "unintentional radiator" by the FCC), there is no legal requirement for certifcation. There are legal requirements for what it is allowed to emit, but it up to you how to make sure your device works within the rules.
You can simply sell a unintentionally radiating device without testing. However, if someone files a complaint and the device is found to exceed the legal radiation limits, you're in deep doodoo. If you had the device tested by a accredited test lab and they determined it was within the limits, your legal case will be much better. The FCC still has the right to force you to withdraw the product and even confiscate every unit out there, but if you can show you followed accepted practices of testing then there will be much less of a issue of punative actions.
Intentional radiators are a different story. You do have to have FCC certification to legally sell one in the United States. When the device is certified, you get a certification ID, and that ID generally has to be indicated somewhere on the outside of the device where others can see it.
In the case of a bluetooth module, most likely the module vendor has gotten the certification for the module. If not, I wouldn't go near it. Even if so though, you are still on the hook for the product as a whole. The module will also be certified with some restrictions, like a specific list of antennas that it is certified with. If you attach a different antenna, for example, the module is no longer certified and you're on your own.
If you're trying to sell a intentionally radiating product, you'd better talk to a expert early in the process. You can wing it a bit with unitnentional radiators, but you really don't want to play games with intentional radiators, even if you're using a certified module that does all the intentional radiating.
It might be a good idea to talk to a testing house. They generally will know all the rules. Just keep in mind they sell testing services, and their answers may a bit biased towards you needing a lot of testing.
Absolutely it is required, I assume you are talking about being a radio in the 915Mhz band? In which case you will be an intentional radiator and will need to go through compliance test at an actual test location, and register the device to get an FCC ID, which you must then apply to your product.
The specific section you are interested in is FCC section 15.249, here is some more information about it from the FCC.
Here are some good app notes, one from TI the other from Semtech.
You'll see that there are also power limits for simple transmission, and higher power limits if you're using spread spectrum.
You will also of course have to pass FCC Class A or B depending on your equipment type.
Edited to answer questions
I'm no FCC or legal expert and you should probably consult one or more of those and a test house. However if you look here you will see the FCC guidelines to "Determine if device requires authorization under Verification; Declaration of Conformity (DoC); or Certification" Where they say for "Unlicensed transmitters (low power intentional radiators) – see Section 15.201"
Paragraph 2 of that section says
"Except as otherwise exempted in
paragraph (c) of this section and in
§15.23 of this part, all intentional radiators
operating under the provisions of
this part shall be certificated by the
Commission pursuant to the procedures
in subpart J of part 2 of this
chapter prior to marketing"
Now subpart J of part 2 states:
"§ 2.907 Certification.
(a) Certification is an equipment authorization
issued by the Commission,
based on representations and test data
submitted by the applicant."
That's your section saying you must apply and provide data for certification, you can read subpart J to learn more about how that testing should be done. I suppose you could collect the data yourself provided you have access to the tools (and shack or are in a valley somewhere with no interference), and that you can follow an acceptable test procedure. A little more digging will show how you have to apply along with the fees.
Or you could go to a test house and pay them, which is what most people do.
Best Answer
The nrf24l01+ is an IC. The FCC does not certify IC's. If you search around, you can probably find an FCC approve module that uses that particular IC. For small quantities, this may be a very cost effective way to go until FCC certification is in your budget.