I will be honest, I got to the third act without noticing that. I was just running and running every time, so I accidentally clicked on the tile on the map, and I fast-traveled to the merchant, and that blows my mind!
Now, when I find the "Card Stand" tile, I transform that in my outpost to refill the Shard to use the card on each combat tile. =D
Another thing you can do is go back and forth on "Corrupted Treasure" and the "Merchant", so you can sell some HP for Money, and buy some upgrades. I usually do this when I'm about to go to the boss, so I spend everything.

And yes... it was a bit painful to run the whole map every time to do that stuff.