Usually, remedies for shipping damages by postal authorities is governed by postal system specific administrative law type remedies (if any) and not by bare contract law principles.
Usually, reimbursement (at least in excess of postage paid or some other very minimal amount) for packages lost or damaged in shipping is only available from a postal system for a country in the case of packages shipped via that postal system, if the package is expressly insured.
Generally, all other remedies are either contractually waived, or are waived by statute or regulation.
Thus, while Alice might have a duty to cooperate with Bob in securing a remedy from the postal service, in all likelihood, that remedy will be minimal or non-existent.
If the shipping terms in the contract are FOB (Free on Board) China, and the shipment is not insured, Bob probably bears the loss and has no remedy. The question seems to imply that this is the case when it states that:
Alice, living in country A, sold an item to Bob, living in country B.
She sent the item using A Post, tracked but with no insurance (the
sale contract obliged her to ship the item, not deliver).
On the other hand, if the shipping terms in the contract were FOB Customer's Address, then Alice would bear the uninsured loss, and Bob could sue Alice for the loss, reverse payment on a credit card payment, or decline to pay if the goods were shipped on credit.
For what it is worth, shipping goods of any meaningful value via the postal service without insurance would be a quite irregular practice in an international sale of goods situation.
Incidentally, to the extent that there is a dispute between Alice and Bob (e.g. regarding shipping terms or contract formation), the law controlling that dispute is the U.N. Convention of the International Sale of Goods (CISG) and not either Chinese or New Zealand law, because both China and New Zealand are parties to the Convention, and this is an international sale of goods, if it is a business to business purchase rather than a consumer purchase from a business.
The relevant provisions of this treaty state:
Article 66
Loss of or damage to the goods after the risk has passed to the buyer
does not discharge him from his obligation to pay the price, unless
the loss or damage is due to an act or omission of the seller.
Article 67
(1) If the contrnct of sale involves carriage of the goods and the
seller is not bound to hand them over at a particular place, the risk
passes to the buyer when the goods are handed over to the first
carrier for transmission to the buyer in accordance with the contract
of sale. If the seller is bound to hand the goods over to a carrier at
a particular place, the risk does not pass to the buyer until the
goods are handed over to the carrier at that place. The fact that the
seller is authorized to retain documents controlling the disposition
of the goods does not affect the passage of the risk.
(2) Nevertheless, the risk does not pass to the buyer until the goods
are clearly identified to the contract, whether by markings on the
goods, by shipping documents, by notice given to the buyer or
otherwise.
