All I can say is, "Caveat Emptor" - let the buyer beware.
In other words, if you order something you haven't checked out with the supplier, it's your fault.
AmigaKit offered to give you a refund for the drives, but not for shipping. This is entirely normal practice with ANY supplier you care to look at.
The drives were sold as A600/1200 compatible - not Infinitiv compatible, which is entirely different. And, IIRC, ALL Amiga drives are modified PC drives, though I stand to be corrected on that one.
I have used AmigaKit many times, the service is excellent, as standard. If you had asked questions prior to buying, you would have saved both of you a lot of grief.
