To offer customers the option to return an item to post office or post box, you must connect your Australia Post account and have one of the following product codes:
PR – eParcel Post Returns
PR1 – eParcel Post Returns Zonal
PR2 – eParcel Post Returns Zonal CHA
XPR – Express eParcel Post Returns
XPR1 – Express eParcel Post Returns Zonal
XPR2 – Express eParcel Post Returns Zonal CHA
You can check this by going to Settings > Carriers > AusPost Standard Post account settings and seeing if eParcel returns is listed as a shipping option.