We live near Al Saad and have a PO Box not far from the Center (Mega Mart)...    From C Ring, take Salwa to the Center R/A and go right.  You will go through 2 traffic lights that are very close together.  Just past the Al Meera store, take a left.  The Post Office is on the next corner. 

Not sure about a waiting list though... Although I did read in the paper that they are closing several post offices.  Didn't say which ones though.