To think I should pay for missed appointment?

[pelagra]

I got confused about dates and missed a hairdressers appointment. It was a week until I realised, and I felt so guilty I thought I'd never show my face again.
I've now gathered myself together enough to call, apologise and book a new appointment.
I commented to a friend that this would be an expensive trip as I'd be paying for both appointments, and she said I should only pay for one. I am right to offer to pay for both though, aren't I?

my hairdressers 'invites' you to pay if they aren't able to fill the slot - they tweet any last minute availability.
It's a social enterprise, so pretty much pay what you want to anyway - I'd probably pay a bit less than if I'd actually been - then again it's so hard to get an appointment I'm unlikely to forget.
they also send text reminders the day before - don't most hairdressers do that?

I think I would offer to pay because they had set aside that appointment solely for your benefit. If they said not to worry, I would make sure I had a £20 note ready to give as a tip.