If you don't have the stomach to see bitcoin prices fall 20% in a day then you don't know bitcoin and should not be dealing with it!
The whole point of Coinbase is to eliminate that risk. They'll guarantee a price for 10 minutes, Coinbase merchants are in and out of Bitcoin so fast they don't take a trading risk. Coinbase itself dumps the Bitcoins on an exchange within minutes, and pulls cash out of the exchange the same day. At the end of each day, Coinbase deposits dollars to the merchant's account. Nobody in the Coinbase merchant world holds Bitcoins for more than a few minutes.