A supervisor from Planned Parenthood called and left a message on my answering machine. I called back and spoke with her. My letter to the regional office had been forwarded to the clinic she supervises.
She said that one of the practitioners had called in sick that day, but normally they make every effort to not cancel Depo shots since they are such fast appointments. Regardless, she agreed that it was unacceptable for them to not squeeze me in ASAP, and that it was against their normal policy to make me wait so long for a rescheduled appointment.
She thanked me for bringing the situation to my attention, and she was really surprised about how poorly it was handled. She also said she would be reviewing appointment scheduling policies with her staff to try preventing this from happening in the future.
I feel much better knowing that the way I was treated was probably a fluke, and I will continue to go to their clinic as I usually receive pretty decent service.
As a bonus, that whole situation allowed me to find out that their walk-in clinic that is a little out of town is actually pretty well run, and not too busy.