Netflix DVDs for me come from Duluth, GA, about a 3.5 hour drive from here. The turnaround is typically two days. If I mail DVDs on Tuesday, for example, I'll usually receive the next DVDs on Thursday.
The return mailer has a barcode on the back that identifies me. I addition, there is a small window on the back. If you put the DVD in the mailer face down, the barcode on the DVD shows through the window. Thus Netflix can tell who sent the DVD and which DVD was sent without opening the mailer. Clever.
Netflix has mailing centers everywhere (almost).