the immunity quoted only seems to appliy to the letter service including  I suppose when the postie carries a parcel to deliver it. 

The PIO did a review of the "discretionary" compensation and made some recommendations years ago, as it is one of the areas they receive most complaints about. 

it is worth a read as it provides interesting overview and commentary on AP reserved services and its commercial operations generally. Should come in google, or is probably buried on PIO website.

Unfortunately since that time we have seen further large strides towards user pay through the nose approach.