The only safe way is to drop it off at the post office window. The carrier on the street does not have seperate bins in his car or truck to seperate mail into different classes. He may or may not hold a rigid envelope or bubble package seperately until he gets back to the office. It is more likely it will get mixed in with the rest of the mail he or she has collected. The same for collection boxes. The mail from them is just scooped out into tubs. It is not taken out one piece at a time and looked at. No matter how much postage is on it, it may never be seen by human eyes until it jams up the canceling machine. To late. You take your chances.
Good luck. Enough said.