Nintendo, or at least the Japanese Nintendo customer support Twitter (opens in new tab), formally cautioned Pokemon Sword and Shield (opens in new tab) players about buying the wrong version of the upcoming Expansion Pass after many people complained about mixups.
The Pokemon Sword and Shield Expansion Pass (opens in new tab) is split into two versions, much like the games themselves (opens in new tab). Of course, both passes come with the Isle of Armor out today, and the Crown Tundra scheduled for this November. However, they feature slightly different Pokemon, characters, and story elements, and as such the passes are not transferable between the two games.
The main reason this is such an issue is that, as the Nintendo eShop support page (opens in new tab) says, Nintendo will not refund or exchange mistaken purchases. So if you have Pokemon Shield and you accidentally buy the Pokemon Sword Expansion Pass, you're just out $30.
If you search for "Pokemon Expansion Pass" on the eShop, you'll find four listings: the Pokemon Sword Expansion Pass, Pokemon Shield Expansion Pass, and bundles for each that also include the base game. The Expansion Pass listings have basically the same art, and version differences aren't necessarily common knowledge, so it's understandable why some people - perhaps parents trying to buy the Pokemon expansion that their kids want - would get confused.
The easiest way to guarantee that you get the correct pass is to purchase it within the game itself. If you open the main menu in Pokemon Sword or Pokemon Shield, you'll see an Expansion Pass prompt in the bottom-right corner. This will redirect you to the eShop page for the Expansion Pass for your game, leaving no room for mistakes. You don't even have to mess with your cart since it's a direct purchase.
Here are the best Pokemon Shield prices and Pokemon Sword deals (opens in new tab) for Switch.