My endo required neither a letter nor any consent forms. She was content with a brief "interview" about my history, then laid out a treatment plan and wrote the prescriptions.
Apparently I found the one endo who treats her patients like adults. :-)
Edit: forgot to add, I did ask when I called to make the appointment, and the receptionist said that no letter or anything special was needed. I agree with the others, do this, especially if money is tight so you don't waste money on a misunderstanding.
I'd also be sure the endo knows that cost is an issue, they may be able to minimize costs (and some will reduce fees for those with limited resources).