You can only send your parcel to a post office using a parcel collection or parcel locker service. You can sign up for this free service here: https://auspost.com.au/receiving/collection-points/choose-a-post-office-for-deliveries
A parcel collect or locker address will look something like this and can be used with our website:
Address line 1: Parcel Collect 00000 00000
Address line 2: 123 Post office/locker Address Street
Suburb
State
Postcode
Do not send a parcel directly to a post office without including the parcel collect or parcel locker details, as it will be returned and incur a redelivery fee of $20.