Apparently, the process was so simple for you because both you (the buyer) and Overstock (the seller) operate via Coinbase. Clearly, the process would have been less smooth if you had had to use your own wallet.
Now suppose that for this reason, Coinbase (as a placeholder for any service like it) manages to establish itself as the go-to payment provider via BTC.
Then, one day, they decide that since they always have to convert back to USD for the merchant, maybe it would be interesting to allow their customers to just keep a USD balance with them. It reduces their exchange rate risk, and they could pass some of the savings on to you, making this a tempting offer.
Suddenly, BTC is not in the loop any more, though in the end all parties would still be better off (lower fees than in the current credit card system).
Such scenarios are why I'm doubtful that BTC will "take over the world" in the sense that many proponents think. I doubt it will become a predominant means of payment in any industrial nation, and there won't be a Bitcoin-standard (analogous to a gold standard) either.
Let me explicitly clarify: That does not mean that I believe BTC's value will necessarily drop to zero within the next 100 years. It might, but it might not.