Only if your terms of service (including anything in your marketing or advertising) or service agreement stated that you would repay if the flowers were late, or if in hiring you, the customer had indicated that "time was of the essence" and the flowers had to be there by a certain time, and knowing that was a condition of the job, you nonetheless accepted the job. Otherwise, legally you would not seem to have to repay under this situation--though depending on the amount at stake, it may be better to refund the money for customer relations/PR reasons.