1/4" is max thickness and up to an ounce for one 55¢ Forever stamp. If it's rigid you have to add a surcharge/additional ounce stamp (15¢). 18 cards was probably just over 2 ounces, 55¢ + 30¢ (additional ounces) + 15¢ rigid surcharge = $1.00.
ohoffm wrote:
I sent out an 18 card PWE which was very flat and got it back for postage due. I thought on the USPS site it said it could be up to 3 oz and 1/4 thick for a forever stamp. Does anyone know the exact rules?
Someday my cards may double in value and then be worth half of what I paid for them.