I needed to send my Tent to my daughter today, 9kg.
I found this site.. http://www.parcel2go.com/
I did delivery to address next day and sent it from an Access Point (a shop about a mile away) for £7.79 - a 3 day service could be around a fiver.
I'm sure many of you are aware of this site, but thought I'd mention it in case you don't, especially if you sell on ebay.
Richard