For oils, I think how long you wait to post depends a lot on how you paint. If you do not have a lot of thick paint or slower drying paint like titanium white then paintings should be safe to ship in a week or two. You can adjust the length of an auction to accommodate some of this. The paint will continue to oxidize after it is shipped but it should arrive safely.
I ship small boards in glassine envelopes, wrapped in a soft foam sheet (not bubble wrap because it leaves marks) then cardboard on both sides and put it in a padded envelope. I ship mostly to USA from Canada. I include a copy of the customs form and a letter explaining to the buyer how to care for the painting and suggestions on where to get frames. I also include a certificate of authentication. It is important for me to keep the size and weight down while still protecting the painting.