If it's over 13 ozs. it's against the rules to put it in a collection box with only postage stamps on it (because there's no way of tracking it should it contain anything illegal).
These labels are supposed to be on every letter collection box across the country:

For those who do ignore this rule, you're subject to having your parcel returned to sender with this label:

I guess the real question is how strictly the USPS enforces that requirement, but since you wouldn't want to pay $$ in postage only to have the parcel returned to sender, it's best to play by the rules, than attempt to challenge it.