£150 is probably her 2 hrs cost.
It's probably a simple case of either missing out the 1 & a half hour price or not realising there was that price slot available.
To me, premium charge like this for 90 mins suggests they expect you out the door after pop 1.
And your expectation of 90 mins suggests effort required.
I always avoid the financially illiterate and this lady has a bad case of such illiteracy.
Guy