Whenever I have been involved in redundancies, fortunately not often, the letters have had to be sent out by recorded delivery or registered post so that it can be proved that the notice had been served.
In this instance perhaps this has been affected by there being no-one home to sign for/accept the delivery. The usual practice for the Postman to put a card through the door indicating where the letter can be picked up. There would not normally be a charge for this, but perhaps the incorrect postage fee was paid on the letter in the first place. In our neck of the woods, you would have to pay to park or risk the double yellow lines.