La Poste

Open 24x7 (imagine). It's an impressive building (currently largest mail distribution center in France) and located about 4 blocks from the rue Rivolli on rue du Louvre. First class postage to the States will run you about 1 Euro. Easy to use self serve machines make it easy. Very friendly and attentive staff.

When your deadline is on that day to send an application or some other material this is the place to go if you're late as it is open till midnight. I'm a regular and I'm not the only one, the post office is big enough for all of us though and the staff is rather friendly, i guess they can see the pressure in our complexion.