Proof of Purchase codes are non-transferable and intended to provide a method for purchasers to verify a genuine product.
The in-game bonus item that is provided with the proof of purchase code is not considered in the purchase price of the Klei Shop Product and is provided while supplies last. In rare cases that a code is somehow missing or damaged, verified purchasers may be provided the bonus in-game item directly on the game account using the same email address as the purchasing account.
Codes purchased from third party resellers are not supported by Klei Entertainment. More information about resold codes can be found here: https://support.klei.com/hc/en-us/articles/4415786461972-Third-Party-Purchases-Reseller-Information