Post it first class now. You might be OK.
Dave is a good bloke and puts an awful lot into organising Audax events but don't take the mickey, he's got quite enough on his plate without you making it worse. Having said that, he can be extraordinarily accommodating. If I were in your position, I would post it today, first class (or maybe even pay a fortune to get a courier to get that entry to him today). Do bear in mind that this weekend he is running not one but three events - the 3am start from Hailsham 300k and the 6am start from Hailsham 300k and then Le Tour de Didling 100k starting from Midhurst at 8.30am. He'll be feeding all these events, somehow, as well. I'm not sure how he'll manage that, but I do hope he will manage it - because I'm in Le Tour de Didling.........