Do they scan mail slot drop offs at USPS?

I never bring them inside and drop probably 200 mailers a week in the slot because I usually go to post office at night before bed. I've never had an issue with it and I'll get scanned. Don't stress.

I have had packages scanned as quickly as 15m after dropoff, and have had others not get scanned until they travelled across the entire country, only getting marked for "Out for Delivery" with no intermediate steps, just suddenly 6 states away.

It's kind of a crapshoot, especially near holidays. Don't worry too much if they haven't been scanned in yet, especially with things potentially closed tomorrow.

It depends on who is working.

They will be scanned eventually most likely, but sometimes they just load the stuff on the truck and sometimes they scan it at the drop off.