I'm planning on shipping shortbread to another state, and it may take 3-5 days to arrive to the destination. It's a fairly butter-rich cookie: 3 sticks butter, 5 cups flour, 2 cups sugar (about) plus spices. How well would they ship, and how long will they stay after shipping? Could they go rancid?
Shortbread ships well, especially if you make the individual biscuits fairly thick. Walker's should be your guide here: theirs are about 0.75 to 1cm thick, which helps prevent crumbling in transit. Shipping them in a tin is also advised.
Shortbread is also fairly shelf-stable. Freshness time depends on storage conditions, of course, but below 75F they should be good for one to two weeks.