The nonmachinable surcharge went to 12c at the last rate increase so the total for a thick letter under one ounce is 49c. Here's the quote from the USPS site:

Nonmachinable Surcharge: An additional $0.12 is required for items weighing 1 ounce or less with any of the following criteria:

a. Square letters.

b. The height exceeds 6-1/8 inches, or length exceeds 11-1/2 inches, or thickness exceeds 1/4 inch.

c. The length divided by height is less than 1.3 or more than 2.5 (length is the dimension parallel to the address).

d. It has clasps, strings, buttons, or similar closure devices.

e. It is too rigid or contains items such as pens that cause the thickness of the mailpiece to be uneven.

f. It has an address parallel to the shorter dimension of the mailpiece.

I have inquired regularly about a 49c stamp but none exists and there's no plans to produce one. This rate will certainly change with the postal increase planned for early next year and maybe one of the the current odd-value stamps that USPS offers will match the value charged for our "brown chip packets".