Shouldn't your guarantee have an "or else?"
"We guarantee that your carpets, when cleaned with the Rotovac, will be cleaner than any other carpet cleaning method" or what?
What happens if they're not happy with the technicians opinion of "cleaner than any other method?" Is it free? Will he clean it again? If spots return will he also return? What happens if there's a problem or something gets damaged?
A guarantee removes risk.
And, this is a people business. People do business with you, not your Rotovac. Besides, it's you who ensures their absolute delight. What happens if your Rotovac breaks a third of the way through the job or it's one of those jobs where the RV isn't the best tool for the job? I'd build trust in you. Your RV should be a just another fine tool in your box of professional tools.