Up until very recently, I was able to send single cards or sometimes up to about three cards via USPS International Large Envelope/Flats rate. The charge was about $2.30 or so a package vesus the $6.16 rate. Around the beginning of this month I had about 6 packages returned saying that they were parcels not flats. From my shoddy memory, the criteria for Large Envelopes/Flats was that the length and width were something that all bubble mailers (even larger ones) would fall into. But then there's the more stringent criteria. From 1 inch inside the outside borders of the envelope, the thickness can not exceed 3/4 inch and any changes in the thickness can not then vary by more than 1/4. Additionally the package must be bendable. For me my packages were failing on the bendability criteria. I had put the card in oriented like the bubble mailer. What I found though is that you can get cards in oriented the other way and they will pass the bendability test...at least as demonstrated by two people at my Post Office. They put them about halfway over the edge of a counter and bent it. If it bent ok, then we were good to go, if not then go to jail, do not pass go, do not collect $200 (slightly more or less in Canada). Some that I had returned I just sent the $6.16 way to avoid any further problems, but some were sent the other method and I've sent others afterwards using that technique and fingers crossed they seem to be ok again. Now of course I've just jinxed myself and I'll be getting packages sent back...