Sandy, that's the quandry. My rules leave it semi-ambiguous, but I do state that late -arriving packages may result in the person not getting their seeds and that the host is not responsible for late -arriving seeds (the implication being that people won't get seeds that aren't sent in time). That said, I'm trying to make my send-by date accommodate a couple of days of 'breathing room' for any packages ruining a day or two late.
If it weren't that so many people do their planting at the end of January, I'd have no qualms about prolonging the send-in period a bit, but I have to make sure they get to me with time for organizing and sending them back out. :)
I'm going to put an 'arrive-by' date in the rules this time and I'll figure out a better solution for the next swap.