Ignore that code if you are buying over the counter postage at your Post Office you do not need to add that code then, on the items you are posting.
You will always see a lesser amount sent to you in your payout, because eBay take their 12.8% seller fee immediately the item sells.
This sometimes is similar amount to the postage paid so it looks like postage wasn't paid by the buyer. Be assured it was, it's the fees which have been taken when your item sold.
See here, for a full breakdown of fees taken and payout received.
https://www.ebay.co.uk/sh/fin/transactions