Sending mail, real mail, in France is pretty much the same as in other places. On the envelope or package, you include the recipient's name (company name), street address, city and finally the postal code, like this:
Monsieur President de la République
Palais de l'Elysée
55, rue du Faubourg Saint-Honoré
In France, the postal code organization was updated in 1972 to the present day's 5-digit system. According to the National Address Management Service (SNA in French - Service National de l'Adresse), an entity of 100 dedicated experts whose role is to make mail an attractive and efficient means of communication, there are 6,300 postal codes that service France's 36,600 communes.
The first two digits of the postal code represent the department: "69_ _ _" for the Rhône, or "75_ _ _" for Île-de-France", for example.
The last three digits identify the location with greater precision: "75008" for the eighth district of Paris. This is critical for your mail to get to the right place, especially when several different communes share the same name. For example, there are at least 15 communes in France named SAINT-SAUVEUR - without those last three digits, the Post Office wouldn't know whether to send your letter to SAINT-SAUVEUR in the Oise, Haute-Saône or Finistère.
And then there's CEDEX
CEDEX is an acronym which means Courrier d'Entreprise à Distribution EXceptionnelle. As it's name implies, CEDEX is an express mail service that gives preferential treatment to large companies, government or public service offices and other large volume users. For these companies, the last line of their address includes the word "CEDEX".
Only the Post Office can authorize the use of CEDEX, a system whose purpose is to increase the reliability of mail delivery, especially in a B2B context. The SNA reports an estimated 21,000 CEDEX codes for 180,000 companies within France.
And finally, remember to put a stamp on your letter before dropping it in the box. Mailing a 20-gram letter in France (with a 2 to 4-day service) will cost you .56€. Unless you are writing to the President, in which case it's free, an exception which symbolizes the equality and democratic right of all French citizens to communicate with their Head of State, regardless of their financial condition.