I'm mocking up a shopping cart for a hair salon. It's going to end up on their Facebook fan page so I'm restricted to Facebook's tab size of 520px.
Because of this I've decided to use an accordion menu as it keeps everything nice and compact.
A request has just come in to say that if a user chooses a service from the colour menu then they must also choose a service from the Cut & Style menu.
What's the most intuitive way of making sure the user follows this rule?
It only applies in this one case.