according to me, the best option is to pay at the General Post Office on the corniche. there you can pay QTEL and Kahramaa bills as well. there are separate counters for each of them.

whats good about them is that, they are open even in the evening. till 7pm if i am right. and lot of parking space! no hazzles as you would experience at other offices.

and you can enjoy the seaview as well. :-)

hope this helps. good luck