Yep, SF...they basically go
through 3 customs hubs these days from China...SF, Chicago, and NYC.
Once Customs has it, there's no tracking info until they release it to the USPS, hence the status staying at "Origin Post is Preparing Shipment" for quite some time. Once the USPS has it, i'll be treated as First Class Air Mail and will move fairly quickly.
Keep in mind they ship this with hundreds or thousands of other items in a single container and the container needs to be filled to maximum capacity before it leaves China, so that can add a delay if they're waiting for a full load. There's a slim chance it's still sitting in Shenzhen waiting for a full load, but I doubt it, with the shipping rush to get everything out before CNY I doubt there's any containers languishing, though there may be a shortage of flights for the volume they're doing this week, but I wouldn't worry too much about that either, they usually increase the number of scheduled flights this time of year to compensate.
Once the container is full, it's loaded on the plane, with all of the items inside listed on a single manifest. That means it can sometimes take Customs days to go
through and match the manifest with goods shipped when the container arrives, and of course more delays as they inspect items in that container. Customs isn't really on the clock, they can take as long as they want, though it's usually in their best interest to get the stuff on the way quickly.
TL;DR: There's a good chance you'll see it this week if Customs is on the ball, but don't get too freaked out if your status doesn't change for several days.