Something similar happened to a card delivered to us - it had the "underpaid" sticker on it despite the fact there was a stamp. Needless to say, the envelope had been opened. It appears this "promising" bit of post had been taken out of the ordinary system for later perusal with a spurious claim of underpayment, and then delivered as normal.

Having said that our postie seems OK and always comes at the same time each day (although whether you think 11am is an acceptable time for the post to arrive is a different matter).