I might be the one who spoke about post card stamps. I used to just add two stamps to anything that was remotely bulky, but that was getting expensive so that is when I got the post card stamps. I have not had anything sent back ever.
However, a relative had to send DS's book back to use recently, and they put it in a regular envelope with only 3 stamps on it, and it did arrive without asking for more postage. However, it took them 2-3 weeks to deliver it (usually takes 2 days). That really should have been $2.30 at least for package postage, so YMMV.
Sending ATC's is costing me far less than sending tags was.