Actually the fat 8ths should be alot heavier than just the block...there is alot less fabric in the block...But it does also go by thickness...I go inside and they slide them through a slot and if it rubs at all they say it needs the laterger postge,,,the 65 or 66 cents ones...Its really better just to pay the 66 cents and not have anyone have to pay for postge due,,,I was told sometimes they let you get away with it other times no...so lets not take a chance.
My husband is a letter carrier and he says sometimes they collect sometimes not