Best way of handling a special requirement in shopping cart

Question

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.

Answer 1

Note that if users just happen to satisfy this requirement, then they don't need to be bothered with any notification.

If users select a Colour option first then show a message in red as a second line in the itemisation - and disable the Book button until all requirements are satisfied.

Then when the user selects a Cut & Style, remove the message, add the item to the list, and enable the Book button.

For example:

Answer 2

Provide a good guidance, explaining this is mandatory... and disable the book button until everything is ready for submitting. :-)