Molly...Will the seeds be sitting in an outdoor mailbox for any length of time at the 20 degree temps?
If not, they should be fine. I get all kinds of seeds, bulbs, tubers and cuttings sent at what causes a lot of vendors heart palpitations once they see my location. I have a p.o. box so they are never outside here, but some things travel through all kinds of temps from all over the country to get here. I'll order seeds any month of the year and haven't had a problem.