I spoke to my sister about it as she knows her, and my sister said "No cake for her then. Stop bending over backwards for her, she's going to keep saying she doesn't want to spend the money until she gets free cake."
I'm just taking it to heart because you wouldn't think friends would try to extort you. If anything I would say "Here's £70 just in case you need to pick up any little things, any change (which would be a maximum of £8!!) you keep as a thank you" But instead they just want to get the cheapest deal
Unfortunately, people are more self centered these days and don't care if they are taking advantage of a "friend". After all, you should feel honored to do this for her.
I don't mean to sound so negative but people seem to feel entitled to whatever they want. That's why a true friend who doesn't take advantage is so special.
It's up to you. I don't think that you're being unreasonable at all. I think that you're giving her a heck of a deal. Too bad for her if she can't see your kindness for what it is.