Any mail dropped in a collection box is probably NEVER even seen by a postal worker untill the time it is delivered. Even if you write "HAND CANCEL" on it ten times, no one may ever see it until it is delivered unless you take it to the post office. If it is dropped in a collection box, it is dumped from there into a larger box, dumped onto a conver belt, a machine turnes it face up, runs it through a cancel machene and a sorter puts a bar code on it so it goes to the right area, there it goes through another sorter where it is sorted down to the delivery route and machine sorted again down to delivery sequence for the carrier. Even the carrier will probably not see the piece before he gets to your mailbox. That is why almost every piece of mail you get now has a bar code on it. No more sorting in the office for the carrier. Only magazines. It may be his responsibility to hand cancel the stamp if it was missed, every truck has a canceler in it, or bring it back to the post office if they realize it needs more postage. Most often they will not bother. Be nice to your mailman.