You can either email/eBay message the codes or physically mail the code cards to the person who buys them.
@carrot, I would aim for the lowest cost of getting your codes to the buyer. Selling codes can be a big headache, and physically mailing the cards won’t protect you in any way since the buyer can claim they were used. Selling code cards puts you in a disadvantage, because it’s in the grey area of selling on ebay and it won’t provide you much in terms of seller protection.
Just make it very clear the code will only be sent by email and use free shipping. The buyers typically understand this is the standard way of sending codes. I’ve never had an issue.