Look, I’m willing to give you the benefit of the doubt that you’re getting at arbitrage here, and I agree you can blame people for that, but this isn’t arbitrage. This is a dumb faggot crashing out because he made an oversight designing his system and can’t handle it maturely. The coupon was only available to this guy as a Patreon reward. Does that sound like a guy who was likely trying to exploit a loophole? What’s your theory of mind here? Because mine is that this guy—who, again, only gets this discount because he donates money on a monthly basis—thought something like “oh, hey, the store I like is selling a product I like. I’ll buy it. It’s nice that I get [X%] off for being a Patreon supporter!”

A good way for the store owner to have handled this is DM’ing the guy to say “look, I overlooked this, the coupon shouldn’t have worked,” and then depending on exactly how bad this is for his financial position, either eat the damages this time and fix it going forward, or try to get his Patreon member who he already knows is a loyal supporter of his to agree to something else. There is no world in which a passive aggressive meltdown where you imply you could “lose” his order but keep the money is appropriate. Nor is it appropriate to crow victory over having “bullied” your supporter into giving you more money by semi-publicly shaming him over your mistake.