There are pretty common ethical standards about disclosing vulnerabilities privately before disclosing them publicly. I don't see how the obviousness of the vulnerability changes the situation. By warning the company, you give them the opportunity to remedy the problem before announcing to the world that anyone with a laptop can exploit it. Probably they were just hoping that nobody would notice, which is stupid of course, but now they don't have the chance to build up a better wall before the flood of fake orders that could cause real harm to the small businesses whose financial information you disclosed online.
Perhaps I'm being too optimistic about how the company would respond, but I still think it's hard to justify not doing a private disclosure.