Why? Are they totally lazy? Don't they want to offer food customer service?
For everyones information, Australia Post DOES work on Sunday. You can mail anything in a red post box on Sunday before 6pm and you item will start moving to it's destination on Sunday night.
If you are posting something within 100km , your item will arrive on Monday.
Customers absolutely love this, and you will receive a lot better feedback for the tiny bit of effort.
Customers like me, however, will not think very much of those who say something can't be posted till Monday.
If you have postal supplies on hand (this helps, by the way) and it is a small parcel, then you can use the red post box. Have a go and see the difference n your customer service and your DSR's.