Bitcoin price goes up and down all the time. If someone buys something costing 10$, the next day it may cost less or more. So it's a risk for the seller.
That's why shop or merchants supporting bitcoin payment is partnered with a crypto company or services. They need this partnership with a crypto company so that they can instantly convert the bitcoin they will receive from their buyers to fiat.
Here in our country, there are now lots of merchants online that support bitcoin payment. There is a third-party company, a crypto service, that will serve as the payment processor and will instantly convert our bitcoin to fiat after confirming everything.
And shop owners are already aware of those risks if they will consider adding bitcoin as a mode of payment. They sure study the risks.