'Thank you so much for the one time delivery and I really appreciated the flower and plant choices.'
'They look wonderful. Very impressed at how quick they were delivered. Thank you!!!'
'I want to Thank You very much for taking such great care of getting my order to arrive with such speed. Knowing that I will use you folks again should I need to. Thanks again'
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.