I used to use manila envelopes to send my ebay sales out in, but I have switched to the poly ones. I bought them on ebay, and they averaged (with shipping) about $.10 each. I've not received any complaints yet! I recently purchased some poly bubble wrap envelopes to ship the bags out that I sell. I paid just under $.50 (with shipping) per bag (14" x 20" ) . I have successfully sent two of these bags to Germany with no problems. I am now able to send the bags out at just under 1 pound-shipping them in the box put them into the 1-2 pound range, saving me about $4 in shipping. Oh-and taping? Don't really need to do it with the poly envelopes because that flap STICKS!
I figured by the time the manila envelope, ziploc bag, tape and time were all added up, I was at least equal if not ahead!