Ok, I see a bit of the issue now. Let's start with you internalizing what happened last week. This is business. Let that go right now. You can go out of your way to help whoever you choose. But it's only your product line that counts.
The customer was WAY far out of line to even ask you how to fix a broken scale. That was rude and passive aggressive. You make the DRO. Not the scale. You currently offer assistance with functional scales. The moment you determine it's bad, you need to politely but very firmly full stop. Be blunt when necessary.
The supposed "friend" of the customer who cancelled because you wouldn't help fix a BROKEN scale (you don't sell scales!) starts to raise lots of red flags for me. You didn't lose a sale Yuriy. You got lucky. You don't want or need either of those people as customers. There was more trouble coming your way with that I guarantee it. Not taking on bad work, leaves you free to do a dozen more good sales or improve your product.
I've worked in customer service fixing people's computers for the better part of 30 years. You must, quickly, learn how to politely say no or deflect and refuse any work you suspect is going to have a bad outcome or take too much time to be practical. There's good work, and there's crap work. You have to divest yourself from the crap work or stuff that makes you 5% of your income will suddenly be taking up 100% of your time and you simply won't get anything else done and you'll be stressed and miserable all day while trying to get that stuff done and "back" to what you really need to be doing.
The guy who blew up 6 scales with the wrong adapter... let me ask you, does Digikey or AutoMation Direct even respond to people who say "I bought this random electronic component, hooked it to stuff it wasn't designed for, and smoked it I want you to pay for all my dead parts that I bought from somewhere else that I fried." Yeah. That sounds silly doesn't it? That's what the guy is telling you. Ignore him.
You want everyone to be happy with you and that's admirable to strive for, and you can achieve a stunningly high success rate. But there's a few percent that are unreasonable and/or out to USE you. And they'll be passive aggressive and/or make you feel like a jerk for not doing something that's not even in your wheelhouse or is a massive waste of time.
Take a deep breath and objectively look at what you are doing and providing like you are a 3rd party looking at what is happening with both you and the customer and decide if you are doing enough or doing the right thing.
Above all, be firm in doing only the work YOU want to do. It's your business. Your job. And your life. You get to pick what kind of work you do and don't want. If you let the job rule you all the time rather than you rule the job, you'll burn out or go bust or both.
To give more specific advice, sounds like at the top or bottom of your direct list of recommended scales you need to put "No direct support for generic scales from AliExpress or Ebay." And you can feel free to say why. "Too many issues, too many failures, too much support time, too frequently junk. Many of them may work fine but you are on your own."
You may need to beef up that list of scales and go so far as to say, my system may work with any scale but I ONLY provide contact support for these: (List). If you do not have one of these, I'm sorry but no support.
Any maybe if this trend continues, you will need to remove the "contact us" and just instead provide a whole lot of very nice guides for various scales but don't allow people to directly reach you. I'm not saying that you SHOULD do this, but it's certainly an option and not a terrible one. People can't say YOU provided bad support when the offered support (comprehensive guide) was clearly all that was offered before purchase. They could read it before buying.
And you could do both that, and provide a per-incident support charge. Then you'd actually make money for the time you spend directly supporting scales which aren't yours. It would take the sting out of wasting time supporting someone else's products. It's completely fair.
I charge people all the time for troubleshooting stuff from their Internet provider like their routers, modems, cabling, email... these are things that should be supported free from their provider but they can't manage to do it. It's not my job to support their internet connection for free just because they got a computer from me.
Like I said, you need to run this business in a way where you get to live a decent day to day work life too without frequent abuse. Make it happen. I wish I had taken firm control of what work I would take on every day a whole lot earlier than I did.
