From a different point of view:
Why not move up the professional scale a bit? Get a website and only let your customers have access to it. Have some useful information on the site for them - when to do so-and-so. When to do this-n-that. Why you are doing so-n-so and this-n-that. You could even run ads for specials, and maybe even get into a relationship with a local nursery and let them place sale ads on your site. The possibilities are endless.
Also, have you customer's emails addresses and send out monthly info reports. Why worry about grubs? What can they do to your lawn and how your service is going to eliminate that. Why fertilize in March, but not July? Yada, yada, yada.
Also, you could bill by email and if you accept credit cards, they could pay via email.
Don't let some slime ball out maneuver you. Get ahead of the curve.
Regards and good luck,