How do I get a refund for a double charge on plan selection?

I am trying to figure out how to contact someone about a refund on a double charge when selecting a plan. I originally I selected basic (which is what I wanted) but when I selected domain it stated not avail with plan. So I upgraded to the next choice and it also said not avail with plan so…went back to Basic. But I got charged for both. Can’t figure out how to get refunded for the upgraded plan.