When you make transactions with the Coinbase debit card, you are engaging in profit-or-loss trade because the crypto assets and digital assets you select from your coinbase portfolio are sold at the point of purchase and used to buy the goods. This is why Coinbase applies a 2.49% liquidation fee which can be reported to the IRS as a tax paid on your purchases. The story is quite different with stablecoins. Because these coins are pegged to the US dollar and relatively stable compared to other cryptocurrencies, they are not considered profit-or-loss trades. Therefore, no tax is expected on them. That is why the Coinbase card does not charge any liquidation fee when you trade with USDC.