No hidden fees...
You pay 0.4% + gas, with a minimum of $5. That's it. Period.
Example 1
Suppose you want to buy 250 USD Coin (USDC), and let's assume the Ethereum Gas price is $3.
0.4% * $250 = $10
∴ you would pay $250 + $10 + $3 = $263
Example 2
Suppose you want to buy 100 Tether (USDT), and let's assume the Ethereum Gas price is $3 again.
0.4% * $100 = $4 < $5
∴ you would pay $100 + $5 + $3 = $108