Well, it is impossible for the seller to make an ironclad guarantee on the arrival of the product when they are not the one controlling that. What the seller has to do is offer a refund if the product is not received, but cannot influence delivery once it leaves their hands.
The seller can purchase insurance from the shipper so that if the shipper fails to complete the arrangement between themselves and the seller, the seller receives the dollar value of the product. Then the seller can return this to the buyer.
So, the buyer guarantees to deliver payment to the seller. The seller guarantees to the buyer to ship the item. The shipper guarantees to the seller to deliver the item to the buyer. The only difference is that the shipper is able to charge an additional fee before allowing themselves to be obligated to refund monies if they fail to satisfy the agreement. The seller may pass this fee on to the buyer, if they agree.
In this, the seller does have the most risk, since it is not up to them to determine if the failure of delivery was the shipper's fault, an outside factor (theft, unwarranted confiscation, etc), or inability of the buyer to receive the package by their own actions. No matter the determination of the shipper, the seller will be forced by the community to refund the buyer's payment.