Oh dear.
15mins isn't on her profile so I guess she resents doing quickies. Perhaps it makes her feel
dirty and used? 
Not the OP's fault of course because the maid agreed it.
Perhaps she takes a snort before every booking, so by the time she's paid for the coke there's virtually nothing left from the £60 and it's almost like she had sex for free?
These are the type of things that go through a WG's mind.