This has happened very infrequently considering the hundreds of artworks I've sent out, perhaps 3 or 4 times over 20 years, but sadly it's always a possibility. It has only ever been the glass that has smashed, usually because of the rough treatment the courier has dished out.
I do pack my work as securely as I can using several layers of bubblewrap and recycled cardboard. I usually use UPS as my preferred courier and select a 24 hour service. However, if you find that despite my efforts your picture has been damaged in transit, please photograph it as best you can, including the packaging and email the photographs to me, which will help in any claim against the courier company.
There are several choices open to you from that point.
You can return the picture to me as safely and securely as possible and obtain a full refund of the purchase price once I have taken delivery.
You can return the picture to me as safely and securely as possible and I will reframe it and reship it once it has been repaired/redone.
Preferably you can get the artwork reframed at your local high street frame shop and I'll pay a contribution to the cost, to be agreed upon before hand. I think this is the most sensible option, to save risking the courier's rough treatment again.