Even though they are not flat rate any more, you can stick one in a FR envelope and it will still go for $4.05 regardless of the weight. Only restrictions you cannot use a bunch of tape to reinforce the envelope. You have to be able to close and seal the flap; but once it is closed and sealed, you can add a piece of tape to reinforce the seal/flap. There was a heated discussion about this recently and there are some folks who feel it is illegal or morally wrong to do so; but the PO with rare excepting accepts that method of packaging.
BTW, in case you didn't notice, you can order those boxes from the PO online. The are free and the PO will deliver them to your home without charge.
