Be sure to return the item as faulty. It doesn't have to be a major fault. For example, perhaps you found the fan was too loud and on too frequently? Any minor scratches or marks? Packaging arrived tampered with or with signs of damage? That kind of thing.
Assuming you bought your iMac directly from Amaz0n and not one of the marketplace vendors, if you return your item as faulty then you shouldn't have charges deducted from your refund.
Andrew.