According to hour listing you proposed using International Standard.
International Standard according to Royal Mail booklet the maximum I can see is £35.70 for a small parcel up to 2kg. BUT only includes compensation up to £20.
Surely you need higher compensation than £20, and should be using a tracked service, but whether you can avoid using ebay services I do not know, but you should ask again at the post office, giving the size/weight and value to use the correct service and cost.