I (hope I) have learned to mail earlier rather than later.
I love sending packages to Loved Ones, and I always think they will take less time than they actually will. Because of this, I end up paying more than I would if I had just packed it up and hauled it down to my cute little small-town post office, where they address you by name, a few days earlier!
I have, for sure, learned to never use those pre-paid return stickers for returning mail-order stuff. They charge $6-7, or more, for something I can often mail myself for under $3. I need the difference, you see, to pay for my procrastination, above.