OMG..........I am a buyer only but feel so sorry for sellers these days. 

Why do buyers need any sort of guarantee of delivery time.....sellers have no control of the delivery once a parcel is in the hands of the carrier, be it Australia Post or a courier company.

It is bad enough now that ebay sticks their finger in and gives estimated delivery dates which buyers take as gospel that they will have their parcels in that time.

The only good thing that I can see about this scheme is that sellers will have some say in the arrival times although I am sure that ebay will find some way of restricting the seller's free choice.