I sent a plain white envelope late last year containing 13 cards <2 oz for $1.15 US. I stamped it myself, popped it in my mailbox and it got to my customer in Canada in about 14 days.
In return I received 24 cards and this particular vendor paid $2.95 Canadian at the local post office to send a cardboard lined PWE. A marking on the envelope said "0.075 kg"... that is 75 g or just under 3 oz.
Interestingly, that took 24 days through the Christmas/ Boxing Day/ New Years season and the envelope arrived sliced open on the top and left side edges. Nosy postal inspectors on either side, maybe?