Each pony is apparently assigned a unique code, meaning these bags aren't as "blind" as originally intended.
*UPDATE* Codes have apparently rolled around ponychan for a while now, here is an image of all of them.
So, instead of buying 30 bags in an attempt to get a fluttershy, you will just need to identify the code for one! Unfortunately, we only have five known codes. The rest will need to be discovered by you non-American bronies!
Thanks to BB-Shockwave from the Allspark Forums for this info! And -treehugger for all of the codes!