Roblox Roblox.com Banned account after inquiry on use of a purchased credit Internet

My son likes and uses this game. One can purchase enhancements via giftcard (robux credit) and add a balance to an account. We did this. However, if you use the Amazon app, any purchase will not use the robux credit but rather automatically use your credit card specified in Amazon one-click settings. After discovering this we contacted roblox customer service on this issue to see if we could apply the credit. Their response was two days later, which in the meantime Amazon helped us and we figured the only way to use a credit is via the website where we entered the robux credit number, not within the app. Their reply was of a form-letter type saying the account was suspended and my son banned. So the bottom line is we paid for this and now he cannot use it at all and through no fault of his, he cannot play and interact with his friends with this app. This is an unacceptable business practice.