Open the resource configuration.
Create a Tip.
Enter the following parameters:
1. Enter a name.
2. Enter the message text.
3. Select the tip type — Error.
4. Open the Constraints tab.
Add a limit on the total number of shares — 2.
Then it will be impossible to book one or two tickets.
The error text is displayed on a red background when you select options and on the cart screen.
But the Place order button in the shopping cart is inactive.
See it in action: