It's nog a bug, it's a feature. Doordash uses investment money to evaporate competitors, take over the market and then raise prices for restaurant owners once they gain control. It's a tried and proven concept.
Paying part of the meal is part of that strategy. They know full well that large orders and large amounts of transactions cost them more money and they're betting on nobody actually doing this. They're selling products below the cost of production at this point, something that I would argue should not be allowed in ethical capitalism. Investors know fully well what they're investing in, and of not, they've either not kept their responsibility on reading about the company they're investing in, or the company itself is pulling massive investment fraud.
Play shit games, win shit prices. If they don't want to lose money like this, maybe they should have a business strategy that isn't oriented about purposely losing money to bankrupt competitors. They easily could've set a reasonable limit of say $200 dollars to their cheaper transactions but they chose not to.
Would I go full ethical when finding exploits for an inherently unethical company? Would I dutifully report flaws to companies selling "adult supervision" apps used by controlling spouses? Would I give "bank phishing on demand" websites a 90 day trial period? I don't think so. Making such software is perfectly legal (in many jurisdictions) but is rarely ever ethical. Ethics would need to come from two sides for me to consider responsible disclosure. I have flooded several phishing databases with fake information, got some of them over their resource limit and shut down as well, and I don't feel the smallest bit of regret.