not sure about other services except singpost.
the recipient will know how much u pay for the postage (for standard regular mail) as the stamps will be pasted on the parcel. for speedpost, should be the same.. not too sure.
for oversea, u can choose "standard regular mail" or "speedpost", speedpost is a few times more ex. but if u are opting for "standard regular mail", is better that you opted for registered mail (additional $2.30).
you can go to the following link for more information:
http://www.singpost.com.sg/quick_services/quicks_02postage_cal.asp