I don't like customers who expect things for free; I decide if it's free, not them.
Depending on 'the extra' and what mood I'm in I say:
"I can do that, but it's going to cost you a cup of tea and a biscuit and I mean the nice buscuits!"
or, I'll do the extra and wait to see it they offer any extra money.
If it's a length of gutter or something slightly more time consuming, I'll say "do you mind if I stick an extra fiver on the bill?"
In this instance, I think her initial text is very rude so she's already dumped.
I would have replied "If you think that's unreasonable, then please have that extra bit, for free"
Can't waste my time (or my mental health) with people like that.