Most salespeople who become business owners always believe they can sell their way out of any service issue. And service techs who start a business believe they can fix other peoples stuff forever. Both of them are wrong.
You should always leverage services to drive more sales, and leverage sales, to drive more services. In either case, you have done the hard part, getting the client to trust you. If the buy from you or call you for service, they trust you, at least a little.
If they trust your service, they should trust your opinion about buying. If they don’t, take a look at your style, you may need to step up the professionally. If they are buying from you, you should be able to grab those services dollars too. If you are not getting the services, take a look at your resources. you may be hiring cheap ahead of good.
balanced service and support, balanced clients, balanced checkbook.