I agree Jim. I have been back and forth with the window USPS clerks and this is from their own site. Some of them think if it is not bendable it must got First Class Parcel. Depends on who you get but Robert E. is right, you can send them for a first class stamp + nonmachinable surcharge.

From USPS site:

If a mailpiece does not exceed 11-1/2 inches x 6-1/8 inches x 1/4 inch thick and 3.5 ounces, but is rigid, it is classified and priced as a nonmachinable letter (i.e., applicable First-Class Mail postage and $0.17 nonmachinable surcharge apply).

Just like Robert says. If it is valuable or delicate of course I would not recommend this as it is common sense.