kitchen timers.... bomb are made of clear plastic.

I haven't seen this. I have only seen the pictures of the bombs without much detail.

I heard that the "bomb experts" said they were real bombs... but if the pipe didn't have caps on both ends... well, its not a bomb. It would correctly be classified as fireworks.

And we all know that the post office does not deliver packages on time. Even if you pay them to do so. Therefor a timer would be the last thing that you would think of to work.

Further, a 1/2" nipple and two 1/2" caps (smallest size at HomeyDepot) weigh more than 10 ounces (the limit of 6 stamps) And, i am pretty sure, from the photos, that they used a larger pipe size.