charged twice for one month (both monthly and yearly)

I have been using Zoom One Pro and paying monthly. The other day they sent me an email offer for a discount for paying yearly. It was late when I saw it and I was tired, so I put it off to the next day and by then they'd already charged me the monthly rate for February. I signed up for the yearly offer anyway. I thought they would refund the monthly bill for February, but they didn't. So I'm paying for February twice, which I don't like.