Microtransactions in Terraria are summarized as...

The game offers no DLC or Microtransactions. Any updates that have been or will be will remain completely free. There is an official soundtrack you can purchase to further support the developers.

The game offers a 4-Pack which gives a substantial discount in case you want to play the game with multiple friends.

You buy it and that's it. No paid DLC. No microtransactions.