you are paying way way too much for packaging
I sell mostly china on ebay ( as a registered business) I charge about 50p extra for breakables only 25p if not breakable like books, brass, vintage linen
my posting stars are 4.87
mailing bags cost 5-10p on ebay
bubble wrap works out at 23p per metre, mailing boxes are about 12p each for small 6" cubes about 25-40p for 12" and above
by costs for sending say a collectable is about 35p -70p bubble wrapped boxed and with packing peanuts, the 70p would be for large cut glass vase, only multiple items such as tea set would justify packing fees of more than £1
it would be cheaper for you to buy bubble wrap and plain bags, also you can get padded envelopes much cheaper than 80p each
anything more than 35p extra for clothes is just silly
Royal mail is competitive below 2kg and cheapest below 1kg second class upto 750g at £2.20 is unbeatable price wise
obviously if you send 100's of parcels a day you might get better price from a courier
also I assume seller has weighed if item says 2.80 postage I assume it will cost 2.20 and 60p is for packing
if it says £4 i assume it weighs more and will cost 3.50 second class with 50p for packing both reasonable however if it says £4 postage and it cost 2.20 that is 1.80 for packing then that is completely unreasonable and I would expect £1 refund for excess postage