IF YOU renewed it via metrash mobile app then you had to tick one option either via POST or via Police
i.e. if you have selected via POST then the license will come to your p.o.box no. which you have mentioned. option one for fact i know its that way.
and
I assume the second option which i have mentioned below will work in this way, its just my assumption.
If you have selected via Police then you might have received some SMS for renewing show it in the police and they will issue you the new license