When you acquire seeds in the mail it is important that you take receipt of them accordingly and look to store them correctly as quickly as possible. Here are some suggestions for you.
In hot weather, make sure you do not leave your consignment sitting inside your mailbox in direct sunlight. This will raise the temperatures and effectively cook your seeds, making your goods non-viable or, at the very least, vastly decreasing their usability. You should be mindful not to leave your seeds in this condition for more than a few hours.
In cold weather, bring your seeds inside from the mailbox, and let them thaw for 72 hours. Do not take any short cuts and ensure that you wait the full 72 hours. Seeds are often frozen during the winter in their native environment and freezing during travel will not harm them as long as you treat them with care.