So I would like to buy 150 pizzas for $4.99, but when I get the steam transaction box, it shows that 150 pizzas cost $4.99, tax is $0.00, so total due is $4.99. But below that the amount I will be charged is actually $6.82! Why is this so? I don't get it.
That's almost definitely Steam applying an exchange rate based on your location. All the Card Hunter prices are shown in the game in USD. My guess is that your local currency trades at 1 to 0.73 USD, which turns $4.99 US into approximately $6.82.