Coinbase (and others) provide merchant services, whereby they accept a small (1% if I recall) fee to convert BTC into USD, at the current spot price. So users

Coinbase (and others) provide merchant services, whereby they accept a small (1% if I recall) fee to convert BTC into USD, at the current spot price. So users can pay for a product in BTC, and the merchant receives USD at the spot price of when the transaction occurred (minus fees). This eliminates BTC price fluctuation from a merchant who does not want to speculate.