Only allow purchase of a show if package is in cart

Good afternoon Tessitura community!

We would like to only allow people to buy a particular show if they are also purchasing a subscription. 

Is there a way to do this with TNEW so that a package must be in their cart to allow access to the show? 

I had thought of adding a message pricing rule that would trigger after adding a package to the cart that has a link to the show with a promo attached but if that promo got out others may be able to purchase without buying a package.

Any other ideas?

Thanks!