Don't do it with a Rug doc , if you want to offer a carpet cleaning service to your existing window customers team up with an experienced Carpet cleaner , who will have the correct insurance and knows what he is doing and just take a cut out of the job , and 6 jobs a day, forget it !
Not sure where you are based, I am very close to J17 on M6, if you would like to talk about offering a service to your customers and are based East / South Cheshire let me know.
Paul.