As the item has sold you cannot now change or charge the buyer any more for the postage if there is an excess to pay.
You do not have to use eBay's postage. For items where you can see the postage is not covering the cost of the item, just use your own postal service, weigh and measure your item check the price against Royal Mail's prices online, then add the correct price to your listing. Take and post the item at your Post Office.
I have never used eBay's postage labels preferring to use my little village Post Office as I know then the correct charges are made, a receipt is given, all good.
@roseog-84