the shipping cost in Vietnam is so crazy, even u use DHL, UPS, FEDEX, or Vietnamese post office. i used to pay 50$ to send a letter to American till i found a company who is agent of DHL,UPS... their cost seems to be much better, u can try too: Mr Thien:0982734284. but i am not sure how his English.

Around the corner from the post office there is an EMS office. You should try there as the prices are quite good and cheaper than the big names. I ship with them alot oversea's and never had any major issues.

They also have a few diffrent options for the shipping time so if your not in a hurry you can save some money.

As it's only 20kg try contacting the airline about unaccompanied baggage or even excess - BA charge 60 pounds($120 -advance payment online) for that weight of excess (it's slightly more if you pay at the airport) - perhaps there is some similar service with the airline you are using?

Thanks for all your advice. Just in case anyone else has the same problem here's what i found out.
To send a box of your stuff home cheaply through the main post office you have to address it to yourself to avoid higher charges and tax. Found out 20KG will cost about $60 to arrive in about 3 months.
Checked with my travel agent about unaccompanied baggage and with Thai Airways, if you check the bag in at their office near the airport at least a day before your flight it'll cost $4 per KG or $25 per KG if you check in the same day.
Hope this is useful!