due to their thickness (greater than .25" thick) therefore you have to pay the .12¢ non-machinable surcharge. Besides, the machining process involves the mail going through rollers, machines, etc.. and you that can damage or destroy a chip.

Here is a link explaining it:

http://pe.usps.gov/text/DMM/c050.htm#Rau94243

Cheers!

Bill R-4581