Careful. Although I do the same (only charge like $25 for the repair), you need to tell the customer that this is only a temporary repair, as the likelihood of getting an RROD is certain at some time in the future. The repair can last months, years, even days, but it WILL eventually RROD once more. It just depends on several conditions, like usage, temperature, etc.
I would let the customer know this, because any RROD repair is not permanent. But hey, it's better than charging over $100 for the same thing from a repair shop.