Dealing with the dreaded question: why do i pay more for bridal hair than just a hair up?

Hi, I'm a hair stylist and make up artist  that mainly does session work, but broaden my scope for work I've begun taking on private clients for special occasion hair up/make up parties and bridal work. 

I was contacted with regards to a party booking and quoted accordingly for special occasion, non bridal work. But the lady emailed me back and said that actually it was for her as a bride and her bridesmaids, and why would I charge any more because it's still a party booking.

Now I know that I can explain that bridal hair and make up involves a trial which will be extra, but I'm still struggling to find a way to explain diplomatically that wedding hair is a different service from your typical hair up. I already feel I'm quite competitive with my prices and I don't want to be seen to massively undercut the local market. How do you all deal with this question? Thanks in advance!