Again, Postal Employee's showed just how STUPID they are, They overcharged you! What you had was a
NON-MACHINABLE LETTER...
Go to the USPS website and do a search on "Non-Machinable Letter" in Postage rates. It basically states; A Rigid regular sized letter weighing between 1oz.-3.5ozs. that contains a coin or coins has a postage rate of $.62 cents!!!
I have been sending up 1-3 chips that way for several years now!

I think the 62-cents applies to just one ounce. For example, I went to the web site and found this:

Question: I see there soon will be a 62-cent stamp. Is this a new price category?

Answer:No it is not a new price category. The 62-cent stamp offers the convenience of paying postage for 1-ounce nonmachinable letters with a single stamp. The stamp will be available May 19 [2008].

http://www.usps.com/mailpro/2008/marapril/page9.htm

62-cents is 42-cents (one ounce rate) plus 20-cents for the non-machinable surcharge.

Robert