Many times, you will read sweeping statements like this one:
"All tabacs in Paris sell postcards, stamps and Metro tickets, too!"
But don't you believe it for one second, because it's simply not true.
Some tabacs provide some of these services, some can do it all - the problem is that you can't tell until you go inside and ask. Many people working here might not speak English, and if your French is not up to snuff, you might be wasting precious time that would be better spent rushing off to the nearest major monument, or simply sitting in a nice cafe.
What to do?
If you are looking for Metro tickets, the tabacs selling them will have a good-sized, turquoise "RATP" sign plainly visible outside. No sign? No tickets - no matter how much you beg. So you will need to buy them at a Metro or train station from a machine or the agent behind the counter there.
Need Postcards? Buy interesting cards as you wander the streets. The selections in newspaper kiosks are pretty good, followed by souvenir shops, then the stationery shops ("papeteries"). Prices are about the same wherever you go, depending on size of the card and if there are any additional "bells and whistles".
Need Postage? Postage for cards and letters is the same, and is based on weight and destination. There are loads of conveniently-located Post Offices in Paris - look for the yellow-and-blue sign "La Poste". Many agents speak at least some English, especially if you ask politely - but if you start hollering, you might as well be speaking Martian, for all the good it will do you.
You can also use the very handy machines at La Poste, if you want to avoid human contact, buy stamps quickly and be on your way. They are not difficult to operate - but you must know your destination in French - USA is Etats-Unis - since you'll have to enter that in order to proceed (look yours up on GoogleTranslate). Pay using coins or credit card - usually a change machine is nearby, if needed. Look around for the mail slots, and put yours in the right one - "Etranger" for anywhere outside of Paris.
Here is a website with helpful info on La Poste - in English. I think the rates are off by one or two centimes, but the rest is accurate, so far as I know.