All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
ِAll I (Late Grand Mufti Nooh al-Qhodah) know is that the Postal Saving Fund applies Islamic Murabaha in developing funds. If this is the case, then there is no sin in saving with this fund. If not, then look for another institution whose transactions are purely Islamic to avoid falling into doubtful matters. And Allah The Almighty Knows Best.