it may be genuine but the price difference between amazon and Samsung direct shouldn't vary more than a few quid on genuine products for a specific model or range!
In the future always double check the manufactures options and even If the charger is advertised as genuine only buy it on amazon if its SOLD by Amazon and not just fulfilled.
just to ensure you avoid any problems with 3rd party seller's as by that point ot becomes your own responsibility to ensure you find out if it is a legitimate charging brick or cable and not amazon's or Samsung
I would return your unit to amazon and get it replaced or refunded depending on whether it was fulfilled only or actually sold by Amazon
glad nothing has happened to you/and family, your house or device:)