The law:
.
UCC 2-509:
"(1) Where the contract requires or authorizes the seller to ship the goods by carrier
(a) if it does not require him to deliver them at a particular destination [sale "FOB" some place other than destination], the risk of loss passes to the buyer when the goods are duly delivered to the carrier even though the shipment is under reservation (Section 2-505); but
(b) if it does require him to deliver them at a particular destination and the goods are there duly tendered while in the possession of the carrier, the risk of loss passes to the buyer when the goods are there duly so tendered as to enable the buyer to take delivery.
. . .
(4) The provisions of this section are subject to contrary agreement of the parties and to the provisions of this Article on sale on approval (Section 2-327) and on effect of breach on risk of loss (Section 2-510)."
So what does this mean?
Almost always when we buy a knife, the deal is that the seller is to have the knife delivered to our address - a "particular destination" - by a carrier that he selects in packing that he supplies. Under the law applicable to such a set of facts, the risk of loss passes to the buyer only on delivery to that address unless the buyer refuses to accept the delivery OR unless the contract was otherwise. For example, if the seller says, "When you buy from me, risk of loss passes to you, buyer, when I deliver the knife to UPS/USPS/FEdEX," then by paying you have agreed to those terms and the risk of loss passes to you on delivery to the carrier.