Well, you did something wrong definitely. If you will put correctly a code, you will see discounted price and then if you will make payment for discounted price.
If you did click DELETE code after you typed it in, or you refreshed site code need to be inserted again, that is why when you are on stripe site, you see price which you will pay, so you did accept that price.
In this case, we are not making any refunds as this is clearly your mistake.