Also, I am almost always at home, and have said if I'm not, I'm happy to have parcels left behind the hedge - he suggested this and I agreed.
I was a postie for twenty years. I used to do this but wouldn't these days. It is a sackable offence and even if you know and trust the customer if the item goes missing you can be investigated for theft, you can't say "I left it behind the hedge" as that could lose you your job. Also posties have done this and it advertised that no one was home resulting in burgled house and big trouble for postie.
YY to why on the glass FFS. So dangerous if single glazed (our door is too).
Yes this does seem unnecessary.
Is a sign too precious? It is, isn't it. - no, I don't think it is precious at all, you will be helping and informing the delivery person, but things have got so crazy now, if I was a postman still I would take a pic of sign with my pkone just to cover myself.
If you knock I will not hear, if you use the bell, which is in the old part of the house I will.
But how does the delivery person know this? There job is to make sure you here them call.
I have finally got our usual post lady trained, - Nice of you. Did you use biscuits? 
We are allocated something like 90 seconds per delivery and that includes knocking on 2 neighbours and writing cards and traipsing back to put the card through.
Exactly. Postal rounds have changed drastically in recent years. They are much, much bigger due to less actual letters that go through the letterbox and more items such as parcels and signed for that you need the customer to come to the door. In my day you had to wait 3 minutes for an answer but I believe you are told to only wait 90 seconds now. But with waiting a reasonable 'patient' amount of time you can end up taking an average of 3 mins for each item you need to knock for. Given that these days you can have to knock for 50 items a day sometimes that is 2 hours. 3 mins may seem a short time to you but seems an age to the postman.
These days many postmen are on 18 - 27 hour (though expected to work lots more hours when it suits) 3 month contracts and under immense pressure to finish on time and can't claim payment for extra hours taken because they wanted to be patient while customers took their time answering the door. Their contract would probably not be renewed.
So please try and be a bit more understanding if he seems impatient. You may be the 20th house he house stood outside in the rain soaking wet, and it would be his own time ,unpaid, while you saunter down shouting " alright, alright, what's the rush?"
Walk a mile in someone elses shoes and all that.
Having all this. I too get really annoyed at this sort of thing. I try to catch myself and remeber what it was like. Can't blame you though OP. It is a natural reaction.