I ended up buying some of the C5 through eBay a couple of weeks ago, just to try them out, and getting eBay to pay 5% of them meant they cost me $4.58 each, or $5.08 to send priority. I thought they'd be thick paper envelopes, but they're actually card mailers - because of that, I also bought some slightly smaller bubble mailers to put inside them, just to help protect my items a bit. 

I quite like them, and hope they become a permanent product, though I still need to use parcels or the slim box mailers for things that need a touch more room to be protected well. A smaller size for large letters would be great, something that has a chance of fitting in the average letterbox.