Limit Cart Items to 1, if greater than one notify and don't allow purchase

Any advice is appreciated. I’ve created a checkout funnel on a simple site,

I’d like to limit purchases to only one item in the cart. Limiting quantity to one is simple, but I don’t want to lose my shirt because someone is clever enough to add several items to the cart at this special price. One such order has come through already, which was in good faith, and I’ll honor it, but I’d like to catch the problem before a purchase is made.

  1. Buyer adds an item to cart
  2. Cart Opens
  3. Buyer goes back to site and browser for another item and adds a second item to cart
  4. The cart opens and notifies customer that they must remove an item before completing the purchase.
  5. Customer removes an item and completes purchase

Any ideas on how this could be implemented? Thanks