For postpaid user, the code is just follow the menu number. For example.
The menu showed after key in *100#, item no. 9 is "My Account". Then after enter 9, item no. 1 is "Billing Info". Then after enter 1, items listed are

1. Current & outsatnding balance
2. Unbilled payment
3. Payment history

So, the code to access those item quickly will be

OUTSTANDING BALANCE *100*9*1*1#
UNBILLED AMOUNT *100*9*1*2#
PAYMENT HISTORY *100*9*1*3#

You can save in your phonebook to access it quickly.

Just remember to continue it with * and close it with #.