We did this Kingwood Texas Roof Cleaning FREE for the winner of the Raffle for Kings Manor Elementary School where my daughter attends. We gave away 6 driveway cleanings and 1 roof cleaning and this was the winner of the Roof Cleaning.
I just type up what they won on my letterhead. I usually have 30 to 45 day expiration on them. However if someone were to call after the expiration I'd still honor it. I only do them in the off season. When it's cooler and I am a little slower.