Short of hand delivering a package, a seller can NOT gaurantee delivery.
The Seller however can do a few things to protect the shipment.

Insurance, proof of mailing, Tracking, proof of delivery etc.

Anything that I send out with a value of $50 or more is automatically insured.
Most people charge several dollars for shipping.
Unless you are looking to make a profit on shipping, you can buy a bubble envelope, insure it for $50 plus pay the postage for under $2

My feeling is, as a buyer my responsibility is to get payment to the seller.
It is the SELLERS responsibility to deliver the goods.