The postage cost through C&S is, I believe $8.00 versus $7.15 through eBay for the eBay branded satchels. I would go the 'own packaging' route, which works out at $6.48 for the next month and a bit, on eBay.

You are not supposed to use the C&S satchels through eBay. I don't know whether AP check them, but I would be cautious.

if you have sales awaiting satchels, maybe get some brown paper? Buyers tend to get a bit shirty about postage delays.