They probably use a program that calculates with the fluctuating prices of Bitcoin. Whenever you click "order," whatever the price is at that moment in time is the price you will be paying.
I can't ever see Bitcoin becoming an adopted currency, just for the fact it's slow and very volatile. But I can however, see an alt coin becoming an adopted currency. The government keeps talking about making its own digital currency, but then that would just expose all the corruption that's already taking place in it. We wouldn't want to do that now would we???
As someone who has made hundreds of purchases (ranging from notebook computers, to airfare, hotel rooms, software, groceries, clothing, cell phones, other electronics, etc.) using bitcoin as payment, most retailers use a payment processor (most of the time it's Bitpay), which means the vendor is actually receiving $ and not bitcoin.
Try buying a computer from NewEgg and choose bitcoin as payment, you'll be presented with a bitpay checkout process that is good for 30 minutes and as soon as bitcoin is in the mempool (transaction has been broadcasted on the bitcoin network but not confirmed on the blockchain), I would guess Bitpay has already hedged a portion of it through their machine algos trading on the big exchanges.
Ironically, the purchase of fiat (purchasing $ from the bitcoin ATM, or in other words withdrawing $ from the bitcoin ATM) carries the highest "fee" in the form of a wide spread, while most bitcoin ATM's will sell bitcoin for as much as 12% surcharge, the bitcoin ATM will buy bitcoin for as little as 2% lower than official exchange rate, and even if it sounds low, is considered high when there is no noticeable exchange rate differential when purchasing from NewEgg. I suppose you could do much worse if you were dealing with physical gold, buying and selling.
And on that note, I do not consider bitcoin as a good currency. I'm on the camp that it's a form of digital investment asset, which happens to offer the convenience of being able to spend (on coffee) bypassing the need to exchange to fiat.
Also, was checking out Blockfi (a US regulated FinTech company) services, specifically interest bearing accounts for crypto deposits and noticed they offer loans backed by cryptos. Looked it up on YouTube and on an interview they mentioned 0 defaults on those loans, lol. I guess crypto folks don't want to lose what they value as their most important assets.
You can buy a real estate or cars with bitcoin (and a few other cryptos).
https://blockfi.com/home-loans/
https://blockfi.com/auto-loan/