I think USPS.
Also, I've had more than one person ask if they are painted and how to keep the paint nice. They are powdercoated. I'd suggest you DON'T
use Armor All or something like that, something that could leave a film. I have used a few different cleaners, usually I just wipe mine off with a microfiber and use nothing on it.
Recently I've started using Chemical Guys Nonsense Cleaner
But I basically use it on my entire interior, doesn't leave a film or gloss. Looks great and cleans awesome. So it has been cleaned a few times, sprayed onto a microfiber, applied, then wiped with a clean one. Just like the rest of the interior. Nothing special.
If you're having trouble with the threads, shoot a small bit of WD40 or PB blaster in there. My apologies, I was trying to hurry and may have forgotten to chase a few threads, if you spray it up there and try and install it you may have to work it down and up a few times. There may be a tad bit of powder from the powdercoater still in the threads. If there is it is my apologies.
If there are any other problems with the knobs go ahead and give me a PM or an email firstname.lastname@example.org
Hope you all get them really soon!!!!