I will usually work to fix any unit that has the same fault after a previous fix, but any additional parts need to be paid for.
That's the way that I work. If I'm doing a reinstall, I always run a test of the HDD for bad sectors & look for highlighted SMART specs. If there's bad sectors or SMART warnings, I advise the user, that person will often ask me "is it OK to reinstall anyway?". Normally WD or Seagate tools can fix these things if not too bad & I'll go for it, however I advise that I cannot promise that the drive will hold up & that I recharge (or if done for free, $25) if reinstallation is required in less than one year.
To make sure I'm not being jived if it's been slightly over a year, I'll peek in the Event Viewer, usually there were warnings 5-6 months before they called back & sometimes only 2-3 months after I performed the reinstall. I'll get paid, either by agreement or I jack up the price of the drive by that amount. My services are very low cost & in some instances pro-bono (especially virus cleaning), so I don't feel any guilt for charging accordingly.
If by chance I cannot fix a computer or determine what's wrong, including the cost being more than that of the fair value of the computer, I'll say so & there is no shame in it. We're all humans, being imperfect, there's going to be a time that day will happen, if enough opportunities come our way. I'd much rather be honest & not risk damage, there's always someone else who know the issue, just like on this forum. My specialties are reinstalls (Windows & many Linux versions), infection cleaning, drive swaps & RAM changes/upgrades & interior cleaning, including heatsink/CPU cleaning & re-pasting. Some general troubleshooting with networking or printing, but don't re-flash routers with Tomato, DD-WRT or other open source firmware.
I don't do MB or PSU swaps, will swap a CMOS battery & that's it pertaining to that, nor do I reinstall non-MS software that requires a key without a purchase receipt. Keeping things simple holds down my pricing, which in this area, is practically unbeatable. For that reason, I can only accept cash as payment, however I do provide a receipt.