Here's a suggetion.
When I am presented the all mighty question; Hey since you're here and you know something about Technology, I have a problem with my home computer. I am always cordial and I tell them that I offer personal tech service for a fee. This helps weed out the everyday moochers and those who really see that it is a professional service. I also express to them that my costs to them is less expensive than the local Computer shops or to buy another service agreement that they may have let expire. And to date, I have not found anyone expressing to me that I have offended them with the offer.
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