Yeah... some places, ours included, abide by the 3cm rule (if it's parcel mail and not a package, and it's bigger than can be expected to fit in through a mail slot on the door, we're allowed to send it directly to be picked up and just write a card for it) although we only really stick to it on the bike routes because we have much less space for parcel mail than the routes which are delivered by car. That's something that is also inconsistent across the country.