I am a commercial landlord. I wish to dismiss my tenant with a renewal offer. Do I have to do it via a bailiff?

Yes. In accordance with the provisions of the last paragraph of Article L145-9 of the Commercial Code, a notice must be given by extrajudicial act, i.e., by the bailiff.

Any leave issued in any other form is void. The nullity of the leave can only be raised by its recipient (the tenant), even in the absence of any prejudice.