When a customer has notified me that a package has not arrived, I immediately tell them that I will remake their order, and send it as soon as possible. If two of the items are eventually received, {meaning a delay in delivery}, I request that one of them are sent back to me. This has happened on a few occasions, and customers seem fully satisfied with this scenario.
Other possible scenarios could be issuing a full refund, partial refund {this would be reasonable only if you outline it specifically in your policies with legitimate reasons}, or offering an exchange for another equivalent item in your shop.