As a side thought: A lot countries outside of the US don't expect tipping to be the normal standard. (When they do, it usually has more to do with wanting to be like the US, than the need to make the difference between a sustainable income and minimum wage.) It's interesting to see a cultural difference like this spill over into the game and peoples reaction to it.
From my own cultural standpoint, if I said/implied that I crafted for free, and someone gave me 5k for the synth anyway, I would be pleasently surprised and think well of that person. Hey, it's 5k more than I had before and I wasn't expecting anything at all. It would be socially unacceptable for me to be offended or insulted by an insubstanial amount I didn't expect in the first place.
I can see why you would be offened. Though, with so many people from different countries playing this game, maybe this person doesn't share the same cultural identity as you.