One of the collectibles in Red Dead Redemption 2 is Cigarette Cards, which are little picture cards that come packaged in a pack of cigarettes. They are split up into sets, with each set containing twelve cards. There’s a total of 144 in the game, but there are two different ways to get them. Hidden throughout the world are locations for each individual card and all you have to do is pick each one up. The other method involves purchasing Premium Cigarettes, where you will randomly get one of the 144 cards. The only problem with this is that it is completely random and you can even get duplicates, so it might take a while and cost a good amount of money to get all cards.
So, what do you do with these Cigarette Cards? Well, they are a part of the Stranger Mission, Smoking and Other Hobbies. Once you complete Who is Not Without Sin in Chapter 2, you will be able to find this Stranger at Flatneck Station south of Valentine. He talks about the Cigarette Cards and how the sets can be worth a "fortune," so once you find a complete set, he asks you to mail them to him and receive a reward.
As already said, there are twelve sets, with each one containing twelve cards. After finishing a set, you can head to any post office and mail that set to Phineas. There is a reward for mailing your first set, as well as all twelve sets. In addition, each individual set has their own rewards assigned to them, so whenever you submit your first set of cards, you will get two different rewards (all the rewards will be listed below). Also, all of the cards for each set are already assigned a number, so they will be listed in that order.
Note: You can only complete four of the twelve sets before finishing the game, as the other ones require an area only accessible in the Epilogue and beyond.
Tip: Make sure you start the Stranger Mission for this before heading out to find the Cigarette Cards, if you decide to use that method, as it appears some cards do not spawn unless you have started the mission.