Unless you are picking up the item the seller must arrange a courier.
It used to be the case that if the buyer arranged the courier then the transaction fell outside all of eBay money back guarantees. I'd guess that is still the case.
I'm guessing that eBay in some way sets the cost of a courier and if the quoted amount is actually too low then I think that I saw something that said that eBay makes up the difference. I am happy enough buying on eBay but would never sell!
I could be going mad of course! @crewe4444