CLEO Notification is not optional. They must always be notified for ever form submitted. There is no requirement for them to respond, so best to send it delivery receipt required or certified if mailing, so you have proof of delivery.
If you're buying anything on a Form 4, your dealer takes care of submission of fingerprints, photo, and CLEO Notification. Also, all of those can be submitted electronically. The only one that must be mailed is paper fingerprint cards but getting an EFT file of your prints that you can use over and over is the smart move.
There's no point to do paper forms anymore with existence of the ATF's eForm site. Paper forms also take significantly (2-3x or more) longer to be approved.
I have multiple SBR & suppressors from before the stamp was eliminated. Waiting on two (2) Form 4 to be approved for more suppressors currently. Monday will be a month for the first stamp. The last form that I did last year was approved in 3 days. As long as it isn't nearly a year like my first handful of forms.