Firstly, accept it that your bargain buyers have no interest in the price that you bought the yarn for way back when. You aim is to shift stuff you don't want, so accept the couple of quid you're likely to get.
If you're daft enough to try to recoup on postage then accept the fact that you're likely to make no sales.
A 2 kilo parcel costs, at todays rate, £4.41 to send within the UK using Royal Mail.
4 kilos costs £7.06,
6 kilos cost £9.58,
8 kilos cost £11.74,
10 kilos cost £12.61
& 20 kilos cost £14.69.
These are standard parcel rates.
If you have more than 1 cone of yarn to sell, then make it quite clear that you will combine the postage without trying to make a profit on it.
Yarn can be sent in plastic bags - you don't need to use a box for them. It is no insult to the buyer if you send the cones of yarn in extra strong garden refuse bags especially when it is so difficult to dispose of cardboard boxes nowdays.
By sending coned yarn in plastic bags you cannot in all honesty ask for more than 20p or 30p extra packing costs - your buyers will appreciate your honesty & return to buy more from you.
Try to view your listing as a buyer would
If you are selling a very large amount of cones, it is worth finding out before listing the item, how much a courier (found on Ebay) will deliver for. They are ofen half the price of Royal Mail. These couriers come to your home to collect the parcel - how convenient is that? All you have to do is to wrap the parcel up.
Many people selling large amounts (described as a stash or mountain or collection) are too lazy to bother to pack it up to send to a buyer - and these people frequently end up with selling a valuable amount of yarn for a mere pittance because they are depending on someone local to pick it up.