Once upon a time, if you wanted to expedite your progress in a video game you'd slyly enter a cheat code. Nowadays, you cough up your cold hard cash. So is the case in Middle-earth: Shadow of War, which will allow you to buy things like loot chests and XP boosts if you couldn't be bothered earning them the normal way.
"An important aspect of the Nemesis System now comes in forging, customizing and leading your own army of unique Orc followers against the fortresses of Mordor," the announcement reads. "There are different ways to do this, including dominating Orcs by exploring the vast open-world and encountering them as part of Orc society, or players can acquire Orcs and other items through the Market (in-game store)."
You'll be able to purchase loot chests, war chests, XP boosts and bundles of all of these. Loot chests contain gear "of varying rarity", while also sometimes containing XP boosts. War chests reward with new Orc followers, again "of varying rarity" as well as Training Orders which can help customise the Orcs in your command. So it looks like most of what you'll be "buying" with real money is reliant on RNG anyway.
There are two in-game currencies: Mirian and Gold. The latter is awarded in game, but can also be purchased with real money. The announcement stresses that "no content in the game is gated by Gold".
You can read the whole breakdown over here. It's not unusual nowadays for microtransactions to feature in singleplayer games: the first example that comes to mind is Assassin's Creed: Black Flag.