Prevent mixing products in cart

By default, the plugin allows customer to put in their cart at the same time normal products and pre-order products:

Mixed products in cart

In case you wish to deactivate this possibility, all you need to do is to check Prevent mixing products in cart, which you can find in YITH Plugins > Pre-Order > Settings:

pre-order cart options

The customer will see the next message as a result, when trying to add a regular product and a pre-order product to cart:

not possible message