At Wellington College recently they had loads of EOD, which started to cause issues with parking and map numbers. But if the cost only goes up from £10 to £11 or £12, then I guess there isn't a huge need to pre enter other than to "guarantee a map".
Personally I'd have a much bigger difference between the two - say +50% for EOD except perhaps for some of the beginner's courses aimed at locals. With most entry deadlines now a week before the race, surely most orienteers will be able to commit to coming or not when entries close?
Another way to deal with the potential EOD rush is to offer some sort of Map Reservation, but that can make things a little complex as this description for an upcoming event shows:
Enter by Sunday 13 January, after which entry fees increase by £2. After that, up to Closing Date on Wednesday 16 January entry is subject to map availability.
EOD will be available at a further increased entry fee subject to map availability.
“Advance EOD” will be available online until Friday 18 January. Advance EOD means you pay and register (as well as reserve your map) before you arrive on Sunday. If you need to hire a SI Card you will have to visit Registration, but otherwise you only need to discover your start time and can go straight to the Start. Advance EOD start times will be available at the event, and may or may not be available online before that.