You can add it in description, but when a buyer buys two items you just go to MORE ACTIONS and use Send Payment Details,amend the postage as required.
There is no way to load it in advance as postage is based on Size, Weight and Value, and it is not always the case that two items together can be posted more cheaply than two individual packets.
For example one small parcel weighing 1 1/2 Kg would post for £3.10 but two, but putting two together weighing 3 Kg would be £8.99. So two packets would be cheaper than one.
