I usually take mine directly to the Post Office, or at least a neighborhood box I've had good experience with quick turn-around, including one in which on a couple occasions I put the Netflix envelope in on a Monday holiday and my next movie shipped the very next day.
My problem lately has been getting cracked disks. I've had Netflix since early 2002 and in the first several years I had maybe 2-3 unplayable disks total. Just in the past 3-4 months, however, between mine and my GF's account, we've had 5-6, including 3 in the past six weeks!
If I mail in the morning, before noon say, I'll have another one in my mailbox the next day. Assuming it's not a Saturday that I mail to them.
I always take it to the post office. Never a stand alone box out somewhere.
We have a mailing center here in town......