So. Postman Pat right? "Maybe, you can never be sure, there'll be knock, ring, letters through your door."
Is it being suggested, that you can never be sure which of these three options will be chosen by Pat as it would depend on whether a signature was required and whether you have a doorbell.
OR as I believe...
Is it saying you can never be sure whether he'll visit but when he does he follows a strict process of knock, ring AND posting letters. Because that seems unnecessary. If he has normal sized letters that don't require a signature why would he wish to disturb me at all, never mind using the doorbell AND knocking. As a resident of Greendale, this would irritate me.
What do you think?