I use the ebay bransed satchels and the ebay label printing service which is the cheapest way of sending satchels,  $7.15 for 500g and $11.70 for 3kg. They are cheaper because ebay have worked out a contract with AP much the same as any other business who send a lot of parcels.. 

I only add SOD if the item is worth more than $50, I have only ever had one ebay satchel go missing since I started using them back at the start.  As long as they are lodged over the counter and you have a docket witht the tracking number on you have your proof of posting as the details can be accesed on line.

Ebay complaints are a different matter as you have to prove delivery as well as sending and for that you may need SOD bhut I still wouldn't bother as the extra $3 per parcel would certainly put buyers off and I am sure my saving of three bucks on every parcel would counteract the loss of a cople of undelivered items.