Refund for Robucks on Roblox

I recently purchased $20 in Robucks for my son. The Robucks did NOT show in my Roblox account. However, Microsoft STILL took $20 from my bank account. I emailed Mircrosoft requesting a refund or to at LEAST credit the account for the Robucks that I purchased.
I was then emailed stating Microsoft WOULD NOT refund me the $20 because my "refund limit" had been reached within a year. I've spent over $500 buying Robucks for my son, and because of some silly "limit" I can't get a refund???? What type of customer service
is that???? I NEVER RECEIVED THE ROBUCKS!!! THAT I PAID FOR??? Can someone at least give me what I paid for???

:)