I have had this issue as a buyer and a seller.  It appears to happen when you use ping.  Trademe auto fills your membership address without you having to tick any boxes - all good if that address is up to date but not great if you wish to 'send to a friend' and don't realise it has been autofilled.

Sorry to contradict 'L' above but I send a lot of trades barely ever in bags.  Generally in recycled paper - like those brown bags you get at supermarket.  I have never had an issue/ query with this.   You are right about boxes tho.  They are treated differently ( charged volumetrically).   So as Carol says just make sure it would fit into a bag of the stated dimensions - allow for the thickness of the items.  

Hi Yes that is the best way - having nothing listed while you are away.  It is always a bit tricky.  You may be best not to send fixed price offers out because if they are accepted then you have to phaff around packaging and booking them when you are likely already in holiday mode and it will only work if they accept and pay immediately.  I usually make sure all listings are finishing 5 days out from departure and immediately email buyers telling them they need to pay by within 3 days or wait until Y date. Enjoy your time away