Hearthsteel is used through the Battle.net balance and is available in the store both in-game and online.
The developers clarify that most items for houses remain free; only about 36 items will be tied to Hearthsteel. Blizzard emphasizes the balance of time and money: players can earn the main content themselves or purchase it for real money if they want to decorate their homes faster.
The peculiarity of Hearthsteel is that its denominations are selected so that you can buy exactly the items you need without unnecessary leftovers. This distinguishes the currency from typical microtransactions, where players have to buy several sets to get one item, leaving unnecessary amounts.
"The prices for items are selected to match the amounts of Hearthsteel offers, which is convenient for players. By purchasing Hearthsteel for the desired amount, you can purchase the items you need without thinking about which sets to combine to minimize leftovers", Blizzard notes.