Overall, I think that was a great response.

Regarding what to do if they come back my thoughts would be: 1) Bill according to your skill. Even though you are not a pro, you have skill and it is worth something; or, 2) If you don't want to do that, then a step closer to doing it for free would be to agree on a base price for your attendence, something very low, but then agree on another price that they will pay you only after seeing the photos. If they are happy, they pay, if they are not happy, then they don't.

Good luck....