What does "Yes, it’s complimentary." mean in English for Spanish?

In English for Spanish, "Yes, it’s complimentary." means:

Sí, es gratis.

Listen to "Yes, it’s complimentary.": (If you have an HTML5 enabled browser, you can listen to the native audio below)

This is a phrase that is used in the GamesForLanguage English for Spanish Language Game in the following scenes:

• English for Spanish 4, Level 3, Scene 2