Offer a free ap to the untreated customer if he agrees to continue for the year.
Your customer should get an extra--probably a free winterizer at the end of the year--say mid-November. Or a free ap for next year in spring, (that way he cannot cancel, and he avoids any price increase).
Other free extras ...mole control...insecticide for insects or grubs...repair a bad spot. Insects on bushes. Mosquitoes, fleas...