Accepted as meeting the seller's obligation under the contract[1]. It's not saying that a buyer incurs new contractual obligations simply by accepting delivery of goods that weren't part of their original contract.
Are you referring to this in an individual customer aspect? Because it seems that the FTC states quite clearly this is not the case. If a company sends you something you didn't ask for and it's addressed to you, you get to keep it as a free gift. The FTC even uses the exact language "free gift".
This may change somewhat if it is a business transaction, but I think it's quite clear if you get a package addressed to you that you didn't order, there's no contractual obligation.
As an example, if you have ever worked in retail sometimes you get truck shipments of things and there can be extra items you didn't order. Maybe the delivery driver said "hey this was sent to you". Accepting delivery of that doesn't mean you have to pay for it.
[1] https://www.law.cornell.edu/ucc/2/2-601