I see you are already have things in hand. I did this about 3 years ago. Post Office was where I took mine. They pretty much verify that you are you and have provided all the necessary paperwork.

Used to be that the photo provided was pasted directly into the passport - So the photos could only be provided by authorized persons. Now they get printed onto the page directly, so you are just providing an image to be scanned. It still has to meet the size requirements. I went to a professional photographer. One thing he had me do was tilt my eyeglasses a bit to eliminate a reflection.

I too had my application expedited, it doesn't really save much time over the standard service.