actually thicker than 1/4" it could be a large envelope, you can't get DC on that either.
To qualify as a parcel, has to be
Over 3/4" thick OR
Rigid OR
Irregular (lumpy)
but why would you want to put DC on an envelope with one block? 80 cents to see if it gets there, if it doesn't get there the PO will not do anything for you.
Very few items get lost in the mail, save up your 80 centses and you will have nice little fund to reimburse the occasional person who does not receive your package.