Always send your items using a tracked service, and obtain a proof of postage receipt. As long as you can provide proof of delivery for the order, you will be covered by seller protection.

If a buyer hasn't received their order and you cannot provide proof of delivery, you will not be entitled to seller protection and will need to refund the buyer. However, you should be able to claim a refund from your shipping provider to cover you as long as you have proof of postage.