The prices quoted by Putney are for the (new) red Prepaid Parcel Post satchels (except the mediums at $127 for 10 should be $127.30 I think). These satchels all have bar codes and individually numbered stickers - my understanding is that you could use these as proof of postage (according to my local PO) but I am sure there will be differing stories. Whether that meets paypal requirements - I have no idea!