Any packaging cost ( are you really charging for taking items to the Post Office ! ? ) can be added into your starting price.
Simple Delivery is now mandatory for all Private Sellers.
You can use your own postage service / costs, but unless you are able to enter the tracking number for those items, your payouts will be held for a minimum of 14 days before release.
Be aware, if you do use SD then if an item is lost or damaged in transit, it's eBay who pay the refund not you, you get to keep that payment, so there are some bonuses! 🤔
See here what Simple Delivery entails:
https://www.ebay.co.uk/sellercentre/postage/simple-delivery?mkevt=1&mkpid=2&emsid=0&mkcid=8&bu=43952...
@loyde68