I am not going to lie to you. Sending anything from Cambodia is far too expensive to even consider buying and sending home. If you really do need to buy those 30 bags and 10 t-shirts and 100 pairs of earrings, consider either not doing it or accept the fact that you will need to carry it with you until you head into Thailand, perhaps Vietnam, or until you head home. Yes, it is that expensive.
The Cambodian post office system will happily send a letter to Australia via airmail. We sent one from Siem Reap, Cambodia to Sydney, Australia. It cost around $1.80 USD and took close to two weeks to arrive. At least the letter arrived as I did have my doubts.
As for sending parcels, this is where it gets really expensive. I am not sure of the cost of sending a parcel via Cambodia's local post system for Khmers but for us tourists, we were stuck with the EMS system.
EMS is not cheap. For example, a 5kg box sent from Cambodia to Australia will cost you $70 USD plus the box itself. That same 5kg box will cost you $107 USD to send to the USA and $93 USD to send to the United Kingdom.
So what can you do? Well, not much really. We visited the DHL Express shop front and another courier service both in Siem Reap and were shocked at the prices they quoted us. I can't remember the prices exactly but they were more than EMS and approached the $150 USD mark.
Think before you buy in Cambodia.