The best way to find-out about how your customers feel about your service, is to just call them. So many people/businesses today hide behind technology and/or impersonal letters, that a REAL phone call is unheard of.
Set aside some time everyday, make a list of all your customers and start calling. Make the call very informal. Tell them you just wanted to make sure that they were happy with your service and ask them what you can do to serve them better. End the call with a sincere "Thank You" for their time. And mention that you also offer late summer and early fall services to get their lawn ready for winter. If they're interested, you can stop by to give them a quote.
The more you can talk and meet with your customers, the more they will get to know you. And everyone knows that "people buy from people that they know."