I would like to know why as well ebay would you to export them to mypost as there is no postage selected in ebay. When I download the parcels for mypost there the local pickup ones are . Only way to stop it  would be to mark as picked before downloading in mypost which is not possible sometimes for obvious reasons. 

It has only happened twice and on a friday as I like to get any last minute parcels out I sent out a local pickup item without checking ebay first for postage paid.  Address was 5 hours away so unlikely local pickup for a $6 item and I  had not been caught out for sometime I sent it without checking. Buyer never answered my message over the weekend and post office still had it this morning although it was dropped off well before 4pm.