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!