Two more suggestions. I had a customer that wanted an organic program. We agreed to ten minutes of hand weeding at each fertilizer service. Five times per year. Later he agreed to a bit of spot spray clean up in October, if it was low-toxicity.
In addition...you may wish to add an overseeding each spring and fall with one of the extra-dense sports turf type of Kentucky bluegrass cultivars. Read the turfgrass seed ads very carefully. You want a variety that claims to be "very dense" and that claims to be resistant to poa annua invasion. Call your favorite seed company in Oregon and find out which of their grasses (in their opinion) would be most resistant to weed invasion.
Bluestone for instance.
Naturally, you have to charge the customer whatever it costs, and add your profit. Be sure to point out how your system is better than any other company.