You still have to enter the postage. However postage is paid, the buyer needs to know the cost whether they supply the label or they pay to you and you buy.
You do realise that however paid, the buyer only pays what is on the listing, if wrong then you have to pay any extra.
I do not know why you cannot charge the buyer and buy postage yourself, simple postage is hardly simple if you do not understand it.
Are you sure you are properly registered to sell and get paid? https://www.ebay.co.uk/help/selling/selling/start-selling-ebay?id=4081&st=3&pos=2&query=Start%20sell...