The OP said that it just fitted in the lock but badly, if that had been used to pick the lock it would have been found in the lock as the user got it jammed behind a wafer,
Its a bit of trash someone has found on the path and put on top of your post box, it happens all the time, find somthing on the path and put it on top of somthing that can be seen, basically so its not on the floor,
It could have been somthing from the flor that the postman had got stuck on the bottom of his bag then put his bag on top of your post box while delivering mail,
It could have been flicked up from the road by a car or simply somthing someone was sick of carrying around in their pocket,
The answer to your questions are no its not a lockpick, its not been used for picking your lock and no dont worry about it, if you want for very little money you can buy a tubular lock to replace the wafer one you have,