To my great relief I found that I could renew my passport on-line without even going to the p.o. (I don't know about your p.o. but ours has a window for p.ports and people wait there for hours).
So I sent it in, and in 5 weeks or so I got my new one and my old one back with it.
My only lament? Open the two passports and compare the pics side by side...wah