Don't bother with PL yet.
Monitor Hotwire (I think they have a TripWatcher feature to monitor the price) for the dates and vehicle type until a week prior. A good rate should develop. It is peak family/van time period before school resumes so maybe not the cheapest but should be cheaper.

You can book a cancellable reservation at one of hte agency while you monitor Hotwire. You have to wait until 330 days in advance before you can obtain a quote from Hotwire.