Which phrase is not a clear way to indicate that installation incurs an extra charge?

The phrase "Installation fee may apply" is less definitive compared to the other options. It introduces uncertainty by using the word "may," suggesting that there might or might not be an installation fee. This vagueness can lead to confusion for the customer regarding whether they will definitely incur an additional cost for installation.

In contrast, the other phrases such as "Installation at extra cost," "Plus installation," and "Installation extra" clearly communicate that there will be an additional charge for the installation. These phrases are direct and leave little room for interpretation, helping to manage customer expectations more effectively. Clear communication of costs is essential in home improvement sales to ensure that customers understand the total expenses they might face.