Is the expression for free incorrect grammar?

The expression "for free" is not grammatically incorrect, but it is often considered informal.

Here's a breakdown:

* "For free" is informal: It's commonly used in everyday conversation and casual writing.

* "Free of charge" is more formal: This is the preferred option in formal writing or when addressing a professional audience.

* "Free" can be used alone: You can simply say "This is free."

Examples:

* Informal: "I got the coffee for free."

* Formal: "The service is provided free of charge."

* Simple: "This item is free."

Ultimately, the best choice depends on the context and desired level of formality.