You're also assuming here that someone is going to keep the package, even knowing that it isn't theirs. That isn't putting much faith in people to do the right thing. I mean, you never know if someone will do the right thing or not here. Assuming that they do, and you follow everyone's advice here and make Costco send you out a replacement Mac for the one that was lost, what happens when that person gets you back the lost Mac? Wouldn't that make you the thief for having two separate Macs, one of which was paid for?
But as others have said, there is nothing that Apple will do on this, because they didn't directly sell you the Mac. Costco did. And Costco is normally upon this and what to do, especially with not only wanting to keep the sale (that's a huge chunk going to them), but also in keeping your MEMBERSHIP as well. If they are telling you that there is nothing that can be done, you need to escalate that issue up higher in their customer service department, because that is something that they are responsible for. If they could not deliver a product they sold to you in good faith, then they are obligated to either send you a replacement unit, or refund your money. You need to take this up directly with Costco.
BL.