'Thank you for prompt delivery'
'Thank you for delivering the flowers in a timely manner ! It was last minute ordering and was very easy to figure out . Thanks again for assistance in this manner,'
'Just talked to my sister and my mother received the flowers and the arrangement is beautiful. Thanks so much for the flowers and the very good service!'
You won't be able to dismiss this by usual means (escape or click button), but you can close it programatically based on user choices or actions.