Lady not charging enough for Christmas window

[Christmaswindows123]

Someone advertised window painting for decorative Christmas windows. We had a bit of chat back and forth and agreed for her to come. She charges £65. She’s been here for five hours and is coming back tomorrow and the level of detail is WAY better than I expected and the amount of effort is way more. I can give a bit more happily and I’ve told her she’s not charging enough, we are talking central London stop in the street and say OMG how amazing level.

what do I do without offending her?

when I said she wasn’t charging enough she said she was getting back into it and happy to try but I feel like I’m ripping her off.

should I try and give her more money?

Maybe give her more as a ‘tip’, because you were so pleased with her work?

I'd offer more, but I'd also sing her praises on social media so she builds up a client base

This is so lovely I agree with pp give as a ‘tip’

ps can you share a pic when it’s done?

Yes big tip and social media feedback

If you check out the freelance rates for creative roles, using various checkers online, you'll probably find that typical rates for comparable work are something like £50 per hour or more.
So I guess that, once you move away from the crazily low rate she is charging, you would have to offer substantially more, rather than just a token uplift. Are you ready/able to do that? If not, perhaps better to stick with her rate, rather than offering something tokenistic in addition.

Bear in mind that the minimum wage isn't a credible starting point - freelancers get none of the benefits (pension and NI contributions, sick pay, etc) or continuity of work that employees have, so their hourly rate has to reflect this