when the invoice is created, the rate is locked in for 15 minutes.  So buyers and merchants do not assume any volatility as long as the transaction can be paid within the time window.  This allows commerce to take place in a volatile market, with no risk to the merchant of the volatility.  BitPay assumes the risk for up to 15 minutes.  Although most people pay within the first 1-2 minutes, so the risk window is shorter.