Take care Mark your raising expectations with your customers and might be opening yourself to some issues. If your happy dealing with this type of customer then its a great move.
I would offer a free call out instead as this gives you space if the stain will not come out.
Lets look at two situations one where you give the certificate and one where you give the call out. the first is that you are saying you applied the product and not guarantee it, which in the customers eyes is the same thing. you get called out and cant remove the satin so whats the next step for the customer thats paid good money for this protection? or second you get the call and go free of charge to remove the stain and you cant remove it, but this time your the hero.
The customer will talk to her friend about both situations which do you want her to tell her friends?
wear is a real issue with protection and if not clean right way using the right product (wool safe), or topped up after three cleans will not work.
Thanks for bring this up, as I am working on some images for "Wear Age" on sofas and you given me and idea to expand this to carpets as well.