As the envelope is made of paper, a mathematical model of its deformation would be an isometric coordinate transformation and thus must have non-zero Gauss curvature almost everywhere; that restriction is clearly violated by the cited solution of the teabag problem.  

I already pointed out in a comment, that there was an article in the German issue (Spektrum der Wissenschaft, June 1995) of Scientific American, where the topic has been discussed [here (in German)](http://www.spektrum.de/magazin/polyederkissen/822369).  
Maybe the same topic has also been treated a few month earlier in Scientific American.