Accidental purchase that I can't afford

Hello, I accidentally purchased an in-app addon, and did not have enough money for it. I was clicking through the buttons, figuring it would ask me which payment method I would choose, and would warn me if I didn't have enough funds. It did neither of those things. Because I purchased it on my phone, the support team of said app claimed that they could not do anything about it. I do not currently have a different payment method to add and cannot download any apps, even free ones. Since I can't complete the purchase, I can't request a refund through Apple. What should I do?