What do you do if your hair stylists charges you more than agreed for the keratin?

I went to Claudia's Hair Salon and her employee (Layla) told me on the phone that they have a special offer for keratin, 100 pounds for long hair. However after the treatment and everything was done she charged me 150!! I told her that's not what we agreed on but she kept on insisting that my hair is too long and not the 'regular' long. I didn't wanna start arguing in front of the whole salon over the price so I just paid. Was I wrong to pay? Should I have insisted on the agreed on price?