You could just put it on the invoice, just write 1 year guarantee on mold not returning or 1.5 years, etc.... It would be a hard thing to upsell as most people here don't wash their homes every year, usually 2-5 years and some have never had their homes washed.
Just think about it for a minute, You want to guarantee you will not be back there to help them again for so long?
What if there was a teflon coating that could be installed into the duct, vents and on the fan blades that grease would not stick to along with the filters, all the grease would either blow out of the system or just slide down into the grease containers. What would that do to your income?
What do you think a guarantee on siding would to to your income or other people's income. Down here, there have been lots of things tried, stuff online, in stores, etc... Down here in the high humidity, salt air from the Gulf of Mexico, it is just a maintenance cleaning every so often. Lots of products say that they can do this and that and you spend your money but it is another thing you just threw money away on. Maybe in certain climates it might work but here, nothing has worked so far.