It's impossible nowadays to use Coinbase for anything without KYC verification. Also, Coinbase could end up blocking transactions similar to how BitPay has been blocking transactions recently. They're not going to do anything that their banking partner would force to cut ties with them, so they'll rather kick you out before any 'damage' is done.

While Coinbase doesn't charge any fees for their ecommerce services, you will have to pay a Bitcoin network fee to send the coins from your ecommerce account to your regular Coinbase account, and then pay another fee to convert to fiat, and then likely another fee to have the fiat be transferred to your bank account. In other words, their 'zero fee' service is just a gimmick.