whenever I have a problem with a vendor and have paid with Visa, I write a letter to the vendor stating the problems (in your case part broken and charged twice) and call Visa to decline payment. I will return the part registered and insured and fill out the form that Visa will send me. I do not pay Visa the disputed charges as I do not owe that money (not even the restocking for a broken part) but insist on Visa not paying the vendor either. (They collect from you first before they pay.) Has always worked for me and is the main reason why I pay with credit card whenever I have doubt about a vendor.
You should have no trouble getting the 2 charges reversed BY VISA, not the vendor, if you have proper documentation.
PS: I am no lawyer, but next time just state the facts about your experience, which we all appreciate.