It won't help for this year, but next year, phrase it as an "early bird discount". If they have the fee in x amount of time the price is "discounted" to whater it really costs per Scout. The "regular" price includes the late fee. Somehow, telling them they're getting a deal makes people more willing to get things in on time (somtimes).
Other things to try - "save the date" type reminders 4 or 3 months early. Time speeds by for people - it feels like things you know to expect (like annual dues) sneak up on you. Make sure dues/recharter deadlines are on the annual planning calendar, and published where parents can see them.
Some years we offer payment plans, allowing parents to pay in installments, but they pay a premium to do so.