It doesn't "come from Amazon" unless it plainly states "Sold By" and "Shipped by" Amazon in the online order form a purchaser has to fill out to complete his transaction. If Amazon stocks it and ships it, then, yes, it's fair to say it "came from Amazon," no question about that, even if Amazon is not the supplier, etc. But this case has the hallmarks of an Amazon third-party reseller, in which case Amazon is the front end for the supplier and shipper, only. Amazon never touches the items sold and shipped by third-party resellers!
Since the items ship from the reseller, not Amazon, in this case, then of course Amazon can in no way vouch for or verify the item before it is shipped to the customer. Amazon in this case is just the front end of the transaction, and other than that has nothing whatever to do with the transaction particulars itself. Out of the hundreds of Amazon purchases I've made over the last decade, all but two or three of them were Prime, sold and shipped by Amazon. I've never had a problem, accordingly, which does not surprise me.
I've seen this ploy before, as I'm sure many of us have(!)--only with rocks carefully weighted to substitute for the missing merchandise shipping poundage instead of a bag of salt!!!!! That one is a new one on me...😉
But it is precisely why when I use Amazon it's only Prime, shipped by and sold by Amazon that gets my dollar! Oh, I'll wrap this up by stating that on the very few items I purchased from a third-party reseller through Amazon, it was because I wasn't paying attention to the order when I placed it...😉 However, I had no problems there, either, I'm glad to say.
In the event that this guy was indeed being forthright in his description here, I have nothing but sympathy for him, as it isn't any fun when you realize you've been robbed! Grrrr-r-r-rr. It's happened to me a couple of times, I'll confess, but not with Amazon, fortunately. Yes, most of the third-party resellers that use Amazon to collect their sales and purchase orders and handle their customer feedback do a decent job, generally, from what I have read in other reports. But if you can buy what you want "sold and shipped" by Amazon, instead of a third-party reseller, then Amazon is responsible for shipping you what you ordered, period. That's the route I recommend!