It is impermissible
to open a postal savings account as described in the question. This is because it is from among the forms of Riba (usury) prohibited by the Qur'an, Sunnah (acts, sayings or approvals of the Prophet) and the Ijma` (consensus of scholars). Any prizes based on these accounts or others are likewise prohibited because what is based on something prohibited is itself prohibited. (Part No. 11; Page No. 101) May Allah grant us success and may Allah's Peace and Blessings be upon our Prophet, his family and Companions.