Right here in Orange County, California. The Czech Republic is a long ways away from here, too. I imagine the Unites States Postal Service has a big hub up in Los Angeles, which I'm sure the terminal annex here in Santa Ana gets to them that day, after the sorting and whatever else it takes to get mail on its way is done. Little bar codes and such. L.A. is 34 miles from Santa Ana. LAX is about 40 miles from Santa Ana. I think there terminal annex was moved to South Central L.A. just north of Watts a few years back. The one in downtown L.A. is by Union Station. I think it's closed, but I haven't been there in years and it was still open the last time I was there.
I usually mail things like letters right at the post office. That can save a day sometimes if the truck hasn't made the pick up yet. Paying bills and stuff gets put in the mail box or handed to the letter carrier.