Savings in the Post Office – how should he pay zakaah?

Firstly: 

It is not permissible to put money in the Post Office, or in
riba-based banks that take interest and give it to depositors. The one who
deposits his money in either of them will not be free from that; either he
will be consuming riba or paying it, or doing both. 

It was narrated that Abu Juhayfah (may Allaah be pleased with
him) said: The Prophet (peace and blessings of Allaah be upon him)
cursed the one who consumes riba and the one who pays it. 

Narrated by al-Bukhaari, 5032. 

Secondly: 

Whoever owns an amount of money that reaches the nisaab –
even if it is separated, with some in one place and some in another – is
obliged to pay the zakaah on it when one year has passed. The reckoning of
the year begins from the day when the money reaches the nisaab. When one
full hijri year has passed, the zakaah must be paid, at a rate of
one-quarter of one-tenth (2.5 %). 

And Allaah knows best.