If, when you list your item you are choosing the postal option of 0.00p postage then you are including the postage in with your item.
To be able to adjust the postage to suit after the sale then I believe you have to choose the "not known" option. This was how it used to work I don't know that it has changed in some way recently. There does seem to be a lot of confusion of late, several threads covering the same subject. Perhaps there needs to be some store housekeeping going on where sellers check their listings and perhaps work out the postage on each item they are prepared to post away from home or to change the setting to "Not willing to ship abroad" this way the item will not come up in the search in other countries.