I live in Paris but I'll be in Pau next week.

You'll likely fly into Charles de Gaulle. Check to make sure that your flight to Reunion goes out of the same. If the connection is out of Orly you'll need to take the metro ($5-10?), grab the air france shuttle ($15), or a taxi ($60-75). If you decide to go into Paris, cost will be similar, but make sure that you budget enough time and money to get back. A taxi can take an hour to 1.5 hour if traffic's really bad. The metro can take you an hour or more depending on where you jump off.

You might want to verify that you have all the shots that you need before you show up at immigration. It looks like reunion is easy compared with mainland Africa but double check. You do not want to be turned away upon arrival.

If you're French skills are anything like mine, you might want to pack a phrase book. If you have any other questions, let me know.