Leaving out who the shipper is for a minute, the primary loss would be the seller's (UCC 2-613) in that the loss would be total and the contract would void (so the buyer has not obligation to pay the seller, and the seller's stuff is nevertheless gone). The seller has a remedy against most shippers, so that UPS or Fedex would be liable to covering their loss (assuming buyer has not assume the liability by agreeing to delivery without signature).
However, USPS has special immunity: the government is liable to tort claims, but 28 USC 2680(b) states that
The provisions of this chapter and section 1346(b) of this title shall
not apply to... Any claim arising out of the loss, miscarriage, or
negligent transmission of letters or postal matter
Thanks to Nate Eldredge for raising the case Dolan v. USPS 546 U.S. 481 (2006), where it was held that USPS has very narrowly circumscribed immunity from liability. Under this provision, they are not liable for breaking contents when delivered negligently, but that does not mean that they are not liable for breaking things as a result of their deliveries (they broke Mrs. Dolan, in how they piled stuff on the porch). Although this case does not rule on the meaning of "loss" or "miscarriage", on a plain reading of the terms, theft results in a loss ("loss" is not restricted to "mysterious disappearance"). The court also observes that
losses of the type for which immunity is retained under §2680(b) are
at least to some degree avoidable or compensable through postal
registration and insurance
and the court explains that
one purpose of the FTCA exceptions was to avoid “extending the
coverage of the Act to suits for which adequate remedies
were already available”
(that is, Congress opened the government to tort liability in just those cases where there were no remedies: but there has long been postal insurance).