That’s not strictly true; whilst you can use FCC modular certification in some circumstances (follow the notes in the certification filing), you can’t in others - eg if you have multiple radios in a product. If you are making a product that’s hand-held you may also need to perform SAR testing, which is even more expensive ($5k+) unless you want to turn down the power so the EIRP is <50mW, which gets you into waiver territory.
Also, the certification only covers the intentional emitter portion of the product. You will still need to ensure your product passes the unintentional emitter testing that every electronic device has to adhere to.
Certification labs are generally expensive - often $1000/day - but are generally really good with giving advice of what you need to have sorted before you go there. After the testing is done there’s a documentation fee of generally $2k+. If you’re planning on selling outside the US then you can get CE testing done at the same time and save some money, as there are a lot of common aspects.
Labs I’ve used in the past include db Technology (UK), CKC labs (California) and NW EMC (Oregon). You want to find one that has certified a wifi product in the past, otherwise you’ll end up training the engineer on your dime